RDF Primer — Turtle version

W3C Note in Development @@@Date@@@

— This is a first draft of a document that might have become an interest group note. Up until now, however, the document never got the sufficient momentum to be published, so it should be considered even less than a draft. It is kept here mainly for historical reasons...—

This version:
Latest version:
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(Original, RDF/XML version): Frank Manola, fmanola@acm.org
(Original, RDF/XML version): Eric Miller, W3C, em@w3.org
David Beckett, dave@dajobe.org
Ivan Herman, ivan@w3.org


The Resource Description Framework (RDF) is a language for representing information about resources in the World Wide Web. This Primer is designed to provide the reader with the basic knowledge required to effectively use RDF. It describes how to define RDF vocabularies using the RDF Vocabulary Description Language. It introduces the basic concepts of RDF and describes its Turtle serialization syntax.

The original version of this Primer[RDF_PRIMER] was part of the RDF Recommendation published in February 2004, was based on the RDF/XML serialization syntax of RDF. The text of the original primer has been adapted to Turtle for the purpose of this document, and some of the application examples (that were defined by external bodies in terms of RDF/XML) have been removed.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is an Interest Group Note, developed by the Semantic Web Interest Group.

As of the publication of this Interest Group Note the Semantic Web Interest Group has completed work on this document. Comments on this document may be sent to public-n3-discuss@w3.org (with public archive). Further discussion on this material may be sent to the Semantic Web Interest Group mailing list, semantic-web@w3.org (also with public archive).

Publication as a Working Group Note does not imply endorsement by the W3C Membership. This document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The W3C maintains a list of any patent disclosures related to the original, RDF/XML version of this work. @@maybe something is needed here on Yahoo! vs. Dave?@@@

Table of Contents

1. Introduction

The Resource Description Framework (RDF) is a language for representing information about resources in the World Wide Web. It is particularly intended for representing metadata about Web resources, such as the title, author, and modification date of a Web page, copyright and licensing information about a Web document, or the availability schedule for some shared resource. However, by generalizing the concept of a "Web resource", RDF can also be used to represent information about things that can be identified on the Web, even when they cannot be directly retrieved on the Web. Examples include information about items available from on-line shopping facilities (e.g., information about specifications, prices, and availability), or the description of a Web user's preferences for information delivery.

RDF is intended for situations in which this information needs to be processed by applications, rather than being only displayed to people. RDF provides a common framework for expressing this information so it can be exchanged between applications without loss of meaning. Since it is a common framework, application designers can leverage the availability of common RDF parsers and processing tools. The ability to exchange information between different applications means that the information may be made available to applications other than those for which it was originally created.

RDF is based on the idea of identifying things using Web identifiers (called Uniform Resource Identifiers, or URIs), and describing resources in terms of simple properties and property values. This enables RDF to represent simple statements about resources as a graph of nodes and arcs representing the resources, and their properties and values. To make this discussion somewhat more concrete as soon as possible, the group of statements "there is a Person identified by http://www.w3.org/People/EM/contact#me , whose name is Eric Miller, whose email address is em@w3.org, and whose title is Dr." could be represented as the RDF graph in Figure 1:

An RDF Graph Describing Eric Miller
Figure 1: An RDF Graph Describing Eric Miller

Figure 1 illustrates that RDF uses URIs to identify:

RDF also provides a text-based syntax (called Turtle) for recording and exchanging these graphs. Example 1 is a small chunk of RDF in Turtle syntax corresponding to the graph in Figure 1:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix contact: <http://www.w3.org/2000/10/swap/pim/contact#>.

  rdf:type contact:Person;
  contact:fullName "Eric Miller";
  contact:mailbox <mailto:em@w3.org>;
  contact:personalTitle "Dr.".

Note that this example also contains URIs, as well as properties like mailbox and fullName (in an abbreviated form), and their respective values em@w3.org, and Eric Miller.

Like HTML, this Turtle syntax is machine processable and, using URIs, can link pieces of information across the Web. However, unlike conventional hypertext, RDF URIs can refer to any identifiable thing, including things that may not be directly retrievable on the Web (such as the person Eric Miller). The result is that in addition to describing such things as Web pages, RDF can also describe cars, businesses, people, news events, etc. In addition, RDF properties themselves have URIs, to precisely identify the relationships that exist between the linked items.

The following documents contribute to the specification of RDF:

This Primer is intended to provide an introduction to RDF and describe some existing RDF applications, to help information system designers and application developers understand the features of RDF and how to use them. In particular, the Primer is intended to answer such questions as:

This Primer is a non-normative document, which means that it does not provide a definitive specification of RDF. The examples and other explanatory material in the Primer are provided to help readers understand RDF, but they may not always provide definitive or fully-complete answers. In such cases, the relevant normative parts of the RDF specification should be consulted. To help in doing this, the Primer describes the roles these other documents play in the complete specification of RDF, and provides links pointing to the relevant parts of the normative specifications, at appropriate places in the discussion.

This Primer follows as closely as possible the RDF/XML Version of the Primer [RDF-PRIMER]. RDF/XML provides a serialization based on XML, whereas the Turtle syntax used in this document is different. Both serializations are equivalent in the sense that they can express the same RDF information, and the choice among the two is solely based on personal taste, availability of parser, etc. Tools also exist that convert the two serialization format to one another without any loss of expressivity.

2. Making Statements About Resources

RDF is intended to provide a simple way to make statements about Web resources, e.g., Web pages. This section describes the basic ideas behind the way RDF provides these capabilities (the normative specification describing these concepts is RDF Concepts and Abstract Syntax [RDF-CONCEPTS]).

2.1 Basic Concepts

Imagine trying to state that someone named John Smith created a particular Web page. A straightforward way to state this in a natural language such as English would be in the form of a simple statement such as:

http://www.example.org/index.html has a creator whose value is John Smith

Parts of this statement are emphasized to illustrate that, in order to describe the properties of something, there need to be ways to name, or identify, a number of things:

In this statement, the Web page's URL (Uniform Resource Locator) is used to identify it. In addition, the word "creator" is used to identify the property, and the two words "John Smith" to identify the thing (a person) that is the value of this property.

Other properties of this Web page could be described by writing additional English statements of the same general form, using the URL to identify the page, and words (or other expressions) to identify the properties and their values. For example, the date the page was created, and the language in which the page is written, could be described using the additional statements:

http://www.example.org/index.html has a creation-date whose value is August 16, 1999
http://www.example.org/index.html has a language whose value is English

RDF is based on the idea that the things being described have properties which have values, and that resources can be described by making statements, similar to those above, that specify those properties and values. RDF uses a particular terminology for talking about the various parts of statements. Specifically, the part that identifies the thing the statement is about (the Web page in this example) is called the subject. The part that identifies the property or characteristic of the subject that the statement specifies (creator, creation-date, or language in these examples) is called the predicate, and the part that identifies the value of that property is called the object. So, taking the English statement

http://www.example.org/index.html has a creator whose value is John Smith

the RDF terms for the various parts of the statement are:

However, while English is good for communicating between (English-speaking) humans, RDF is about making machine-processable statements. To make these kinds of statements suitable for processing by machines, two things are needed:

Fortunately, the existing Web architecture provides both these necessary facilities.

As illustrated earlier, the Web already provides one form of identifier, the Uniform Resource Locator (URL). A URL was used in the original example to identify the Web page that John Smith created. A URL is a character string that identifies a Web resource by representing its primary access mechanism (essentially, its network "location"). However, it is also important to be able to record information about many things that, unlike Web pages, do not have network locations or URLs.

The Web provides a more general form of identifier for these purposes, called the Uniform Resource Identifier (URI). URLs are a particular kind of URI. All URIs share the property that different persons or organizations can independently create them, and use them to identify things. However, URIs are not limited to identifying things that have network locations, or use other computer access mechanisms. In fact, a URI can be created to refer to anything that needs to be referred to in a statement, including

Because of this generality, RDF uses URIs as the basis of its mechanism for identifying the subjects, predicates, and objects in statements. To be more precise, RDF uses URI references [URIS]. A URI reference (or URIref) is a URI, together with an optional fragment identifier at the end. For example, the URI reference http://www.example.org/index.html#section2 consists of the URI http://www.example.org/index.html and (separated by the "#" character) the fragment identifier Section2. RDF URIrefs can contain Unicode [UNICODE] characters (see [RDF-CONCEPTS]), allowing many languages to be reflected in URIrefs. RDF defines a resource as anything that is identifiable by a URI reference, so using URIrefs allows RDF to describe practically anything, and to state relationships between such things as well. URIrefs and fragment identifiers are discussed further in Appendix A, and in [RDF-CONCEPTS].

To represent RDF statements in a machine-processable way, RDF normatively uses the Extensible Markup Language [XML], but other syntaxes are also possible. The XML serialization syntax (referrred to as RDF/XML) is described in a separate document [RDF-SYNTAX]. This document uses the Turtle syntax [TURTLE]. An example of Turtle was given in Section 1. Turtle content can contain Unicode [UNICODE] characters, allowing information from many languages to be directly represented. The specific Turtle syntax is defined in [TURTLE]

2.2 The RDF Model

Section 2.1 has introduced RDF's basic statement concepts, the idea of using URI references to identify the things referred to in RDF statements, and Turtle as a machine-processable way to represent RDF statements. With that background, this section describes how RDF uses URIs to make statements about resources. The introduction said that RDF was based on the idea of expressing simple statements about resources, where each statement consists of a subject, a predicate, and an object. In RDF, the English statement:

http://www.example.org/index.html has a creator whose value is John Smith

could be represented by an RDF statement having:

Note how URIrefs are used to identify not only the subject of the original statement, but also the predicate and object, instead of using the words "creator" and "John Smith", respectively (some of the effects of using URIrefs in this way will be discussed later in this section).

RDF models statements as nodes and arcs in a graph. RDF's graph model is defined in [RDF-CONCEPTS]. In this notation, a statement is represented by:

So the RDF statement above would be represented by the graph shown in Figure 2:

Groups of statements are represented by corresponding groups of nodes and arcs. So, to reflect the additional English statements

http://www.example.org/index.html has a creation-date whose value is August 16, 1999
http://www.example.org/index.html has a language whose value is English

in the RDF graph, the graph shown in Figure 3 could be used (using suitable URIrefs to name the properties "creation-date" and "language"):

Figure 3 illustrates that objects in RDF statements may be either URIrefs, or constant values (called literals) represented by character strings, in order to represent certain kinds of property values. (In the case of the predicate http://purl.org/dc/elements/1.1/language the literal is an international standard two-letter code for English.) Literals may not be used as subjects or predicates in RDF statements. In drawing RDF graphs, nodes that are URIrefs are shown as ellipses, while nodes that are literals are shown as boxes. (The simple character string literals used in these examples are called plain literals, to distinguish them from the typed literals to be introduced in Section 2.4. The various kinds of literals that can be used in RDF statements are defined in [RDF-CONCEPTS]. Both plain and typed literals can contain Unicode [UNICODE] characters, allowing information from many languages to be directly represented.)

Sometimes it is not convenient to draw graphs when discussing them, so an alternative way of writing down the statements, called triples, is also used. In the triples notation, each statement in the graph is written as a simple triple of subject, predicate, and object, in that order. For example, the three statements shown in Figure 3 would be written in the triples notation as:

<http://www.example.org/index.html> <http://purl.org/dc/elements/1.1/creator> <http://www.example.org/staffid/85740> .
<http://www.example.org/index.html> <http://www.example.org/terms/creation-date> "August 16, 1999" .
<http://www.example.org/index.html> <http://purl.org/dc/elements/1.1/language> "en" .

Each triple corresponds to a single arc in the graph, complete with the arc's beginning and ending nodes (the subject and object of the statement). Unlike the drawn graph (but like the original statements), the triples notation requires that a node be separately identified for each statement it appears in. So, for example, http://www.example.org/index.html appears three times (once in each triple) in the triples representation of the graph, but only once in the drawn graph. However, the triples represent exactly the same information as the drawn graph, and this is a key point: what is fundamental to RDF is the graph model of the statements. The notation used to represent or depict the graph is secondary.

The full triples notation requires that URI references be written out completely, in angle brackets, which, as the example above illustrates, can result in very long lines on a page. For convenience, the Primer uses a shorthand way of writing triples (the same shorthand is also used in other RDF specifications). This shorthand substitutes a qualified name (or QName) without angle brackets as an abbreviation for a full URI reference . A QName contains a prefix that has been assigned to a namespace URI, followed by a colon, and then a local name. The full URIref is formed from the QName by appending the local name to the namespace URI assigned to the prefix. So, for example, if the QName prefix foo is assigned to the namespace URI http://example.org/somewhere/, then the QName foo:bar is shorthand for the URIref http://example.org/somewhere/bar. Primer examples will also use several "well-known" QName prefixes (without explicitly specifying them each time), defined as follows:

prefix rdf:, namespace URI: http://www.w3.org/1999/02/22-rdf-syntax-ns#
prefix rdfs:, namespace URI: http://www.w3.org/2000/01/rdf-schema#
prefix dc:, namespace URI: http://purl.org/dc/elements/1.1/
prefix owl:, namespace URI: http://www.w3.org/2002/07/owl#
prefix ex:, namespace URI: http://www.example.org/ (or http://www.example.com/)
prefix xsd:, namespace URI: http://www.w3.org/2001/XMLSchema#

Obvious variations on the "example" prefix ex: will also be used as needed in the examples, for instance,

prefix exterms:, namespace URI: http://www.example.org/terms/ (for terms used by an example organization),
prefix exstaff:, namespace URI: http://www.example.org/staffid/ (for the example organization's staff identifiers),
prefix ex2:, namespace URI: http://www.domain2.example.org/ (for a second example organization), and so on.

Using this new shorthand, the previous set of triples can be written as:

ex:index.html  dc:creator             exstaff:85740 .
ex:index.html  exterms:creation-date  "August 16, 1999" .
ex:index.html  dc:language            "en" .

Since RDF uses URIrefs instead of words to name things in statements, RDF refers to a set of URIrefs (particularly a set intended for a specific purpose) as a vocabulary. Often, the URIrefs in such vocabularies are organized so that they can be represented as a set of QNames using a common prefix. That is, a common namespace URIref will be chosen for all terms in a vocabulary, typically a URIref under the control of whoever is defining the vocabulary. URIrefs that are contained in the vocabulary are formed by appending individual local names to the end of the common URIref. This forms a set of URIrefs with a common prefix. For instance, as illustrated by the previous examples, an organization such as example.org might define a vocabulary consisting of URIrefs starting with the prefix http://www.example.org/terms/ for terms it uses in its business, such as "creation-date" or "product", and another vocabulary of URIrefs starting with http://www.example.org/staffid/ to identify its employees. RDF uses this same approach to define its own vocabulary of terms with special meanings in RDF. The URIrefs in this RDF vocabulary all begin with http://www.w3.org/1999/02/22-rdf-syntax-ns#, conventionally associated with the QName prefix rdf:. The RDF Vocabulary Description Language (described in Section 5) defines an additional set of terms having URIrefs that begin with http://www.w3.org/2000/01/rdf-schema#, conventionally associated with the QName prefix rdfs:. (Where a specific QName prefix is commonly used in connection with a given set of terms in this way, the QName prefix itself is sometimes used as the name of the vocabulary. For example, someone might refer to "the rdfs: vocabulary".)

Using common URI prefixes provides a convenient way to organize the URIrefs for a related set of terms. However, this is just a convention. The RDF model only recognizes full URIrefs; it does not "look inside" URIrefs or use any knowledge about their structure. In particular, RDF does not assume there is any relationship between URIrefs just because they have a common leading prefix (see Appendix A for further discussion). Moreover, there is nothing that says that URIrefs with different leading prefixes cannot be considered part of the same vocabulary. A particular organization, process, tool, etc. can define a vocabulary that is significant for it, using URIrefs from any number of other vocabularies as part of its vocabulary.

In addition, sometimes an organization will use a vocabulary's namespace URIref as the URL of a Web resource that provides further information about that vocabulary. For example, as noted earlier, the QName prefix dc: will be used in Primer examples, associated with the namespace URIref http://purl.org/dc/elements/1.1/. In fact, this refers to the Dublin Core vocabulary described in Section 6.1. Accessing this namespace URIref in a Web browser will retrieve additional information about the Dublin Core vocabulary (specifically, an RDF schema).

In the rest of the Primer, the term vocabulary will be used when referring to a set of URIrefs defined for some specific purpose, such as the set of URIrefs defined by RDF for its own use, or the set of URIrefs defined by example.org to identify its employees.

URIrefs from different vocabularies can be freely mixed in RDF graphs. For example, the graph in Figure 3 uses URIrefs from the exterms:, exstaff:, and dc: vocabularies. Also, RDF imposes no restrictions on how many statements using a given URIref as predicate can appear in a graph to describe the same resource. For example, if the resource ex:index.html had been created by the cooperative efforts of several staff members in addition to John Smith, example.org might have written the statements:

ex:index.html  dc:creator  exstaff:85740 .
ex:index.html  dc:creator  exstaff:27354 .
ex:index.html  dc:creator  exstaff:00816 .

These examples of RDF statements begin to illustrate some of the advantages of using URIrefs as RDF's basic way of identifying things. For instance, in the first statement, instead of identifying the creator of the Web page by the character string "John Smith", he has been assigned a URIref, in this case (using a URIref based on his employee number) http://www.example.org/staffid/85740 . An advantage of using a URIref in this case is that the identification of the statement's subject can be more precise. That is, the creator of the page is not the character string "John Smith", or any one of the thousands of people named John Smith, but the particular John Smith associated with that URIref (whoever created the URIref defines the association). Moreover, since there is a URIref to refer to John Smith, he is a full-fledged resource, and additional information can be recorded about him, simply by adding additional RDF statements with John's URIref as the subject. For example, Figure 4 shows some additional statements giving John's name and age.

These examples also illustrate that RDF uses URIrefs as predicates in RDF statements. That is, rather than using character strings (or words) such as "creator" or "name" to identify properties, RDF uses URIrefs. Using URIrefs to identify properties is important for a number of reasons. First, it distinguishes the properties one person may use from different properties someone else may use that would otherwise be identified by the same character string. For instance, in the example in Figure 4, example.org uses "name" to mean someone's full name written out as a character string literal (e.g., "John Smith"), but someone else may intend "name" to mean something different (e.g., the name of a variable in a piece of program text). A program encountering "name" as a property identifier on the Web (or merging data from multiple sources) would not necessarily be able to distinguish these uses. However, if example.org writes http://www.example.org/terms/name for its "name" property, and the other person writes http://www.domain2.example.org/genealogy/terms/name for hers, it is clear that there are distinct properties involved (even if a program cannot automatically determine the distinct meanings). Also, using URIrefs to identify properties enables the properties to be treated as resources themselves. Since properties are resources, additional information can be recorded about them (e.g., the English description of what example.org means by "name"), simply by adding additional RDF statements with the property's URIref as the subject.

Using URIrefs as subjects, predicates, and objects in RDF statements supports the development and use of shared vocabularies on the Web, since people can discover and begin using vocabularies already used by others to describe things, reflecting a shared understanding of those concepts. For example, in the triple

ex:index.html   dc:creator   exstaff:85740 .

the predicate dc:creator, when fully expanded as a URIref, is an unambiguous reference to the "creator" attribute in the Dublin Core metadata attribute set (discussed further in Section 6.1), a widely-used set of attributes (properties) for describing information of all kinds. The writer of this triple is effectively saying that the relationship between the Web page (identified by http://www.example.org/index.html ) and the creator of the page (a distinct person, identified by http://www.example.org/staffid/85740 ) is exactly the concept identified by http://purl.org/dc/elements/1.1/creator. Another person familiar with the Dublin Core vocabulary, or who finds out what dc:creator means (say by looking up its definition on the Web) will know what is meant by this relationship. In addition, based on this understanding, people can write programs to behave in accordance with that meaning when processing triples containing the predicate dc:creator.

Of course, this depends on increasing the general use of URIrefs to refer to things instead of using literals; e.g., using URIrefs like exstaff:85740 and dc:creator instead of character string literals like John Smith and creator. Even then, RDF's use of URIrefs does not solve all identification problems because, for example, people can still use different URIrefs to refer to the same thing. For this reason, it is a good idea to try to use terms from existing vocabularies (such as the Dublin Core) where possible, rather than making up new terms that might overlap with those of some other vocabulary. Appropriate vocabularies for use in specific application areas are being developed all the time, as illustrated by the applications described in Section 6. However, even when synonyms are created, the fact that these different URIrefs are used in the commonly-accessible "Web space" provides the opportunity both to identify equivalences among these different references, and to migrate toward the use of common references.

In addition, it is important to distinguish between any meaning that RDF itself associates with terms (such as dc:creator in the previous example) used in RDF statements and additional, externally-defined meaning that people (or programs written by those people) might associate with those terms. As a language, RDF directly defines only the graph syntax of subject, predicate, and object triples, certain meanings associated with URIrefs in the rdf: vocabulary, and certain other concepts to be described later. These things are normatively defined in [RDF-CONCEPTS] and [RDF-SEMANTICS]. However, RDF does not define the meanings of terms from other vocabularies, such as dc:creator, that might be used in RDF statements. Specific vocabularies will be created, with specific meanings assigned to the URIrefs defined in them, externally to RDF. RDF statements using URIrefs from these vocabularies may convey the specific meanings associated with those terms to people familiar with these vocabularies, or to RDF applications written to process these vocabularies, without conveying any of these meanings to an arbitrary RDF application not specifically written to process these vocabularies.

For example, people can associate meaning with a triple such as

ex:index.html  dc:creator  exstaff:85740 .

based on the meaning they associate with the appearance of the word "creator" as part of the URIref dc:creator, or based on their understanding of the specific definition of dc:creator in the Dublin Core vocabulary. However, as far as an arbitrary RDF application is concerned the triple might as well be something like

fy:joefy.iunm  ed:dsfbups  fytubgg:85740 .

as far as any built-in meaning is concerned. Similarly, any natural language text describing the meaning of dc:creator that might be found on the Web provides no additional meaning that an arbitrary RDF application can directly use.

Of course, URIrefs from a particular vocabulary can be used in RDF statements even though a given application may not be able to associate any special meanings with them. For example, generic RDF software would recognize that the above expression is an RDF statement, that ed:dsfbups is the predicate, and so on. It will simply not associate with the triple any special meaning that the vocabulary developer might have associated with a URIref like ed:dsfbups. Moreover, based on their understanding of a given vocabulary, people can write RDF applications to behave in accordance with the special meanings assigned to URIrefs from that vocabulary, even though that meaning will not be accessible to RDF applications not written in that way.

The result of all this is that RDF provides a way to make statements that applications can more easily process. An application cannot actually "understand" such statements, as noted already, any more than a database system "understands" terms like "employee" or "salary" in processing a query like SELECT NAME FROM EMPLOYEE WHERE SALARY > 35000. However, if an application is appropriately written, it can deal with RDF statements in a way that makes it seem like it does understand them, just as a database system and its applications can do useful work in processing employee and payroll information without understanding "employee" and "payroll". For example, a user could search the Web for all book reviews and create an average rating for each book. Then, the user could put that information back on the Web. Another Web site could take that list of book rating averages and create a "Top Ten Highest Rated Books" page. Here, the availability and use of a shared vocabulary about ratings, and a shared group of URIrefs identifying the books they apply to, allows individuals to build a mutually-understood and increasingly-powerful (as additional contributions are made) "information base" about books on the Web. The same principle applies to the vast amounts of information that people create about thousands of subjects every day on the Web.

RDF statements are similar to a number of other formats for recording information, such as:

and information in these formats can be treated as RDF statements, allowing RDF to be used to integrate data from many sources.

2.3 Structured Property Values and Blank Nodes

Things would be very simple if the only types of information to be recorded about things were obviously in the form of the simple RDF statements illustrated so far. However, most real-world data involves structures that are more complicated than that, at least on the surface. For instance, in the original example, the date the Web page was created is recorded as a single exterms:creation-date property, with a plain literal as its value. However, suppose the value of the exterms:creation-date property needed to record the month, day, and year as separate pieces of information? Or, in the case of John Smith's personal information, suppose John's address was being described. The whole address could be written out as a plain literal, as in the triple

exstaff:85740   exterms:address   "1501 Grant Avenue, Bedford, Massachusetts 01730" .

However, suppose John's address needed to be recorded as a structure consisting of separate street, city, state, and postal code values? How would this be done in RDF?

Structured information like this is represented in RDF by considering the aggregate thing to be described (like John Smith's address) as a resource, and then making statements about that new resource. So, in the RDF graph, in order to break up John Smith's address into its component parts, a new node is created to represent the concept of John Smith's address, with a new URIref to identify it, say http://www.example.org/addressid/85740 (abbreviated as exaddressid:85740). RDF statements (additional arcs and nodes) can then be written with that node as the subject, to represent the additional information, producing the graph shown in Figure 5:

or the triples:

exstaff:85740       exterms:address    exaddressid:85740 .
exaddressid:85740   exterms:street     "1501 Grant Avenue" .
exaddressid:85740   exterms:city       "Bedford" .
exaddressid:85740   exterms:state      "Massachusetts" .
exaddressid:85740   exterms:postalCode "01730" .

This way of representing structured information in RDF can involve generating numerous "intermediate" URIrefs such as exaddressid:85740 to represent aggregate concepts such as John's address. Such concepts may never need to be referred to directly from outside a particular graph, and hence may not require "universal" identifiers. In addition, in the drawing of the graph representing the group of statements shown in Figure 5, the URIref assigned to identify "John Smith's address" is not really needed, since the graph could just as easily have been drawn as in Figure 6:

Figure 6, which is a perfectly good RDF graph, uses a node without a URIref to stand for the concept of "John Smith's address". This blank node serves its purpose in the drawing without needing a URIref, since the node itself provides the necessary connectivity between the various other parts of the graph. (Blank nodes were called anonymous resources in [RDF-MS].) However, some form of explicit identifier for that node is needed in order to represent this graph as triples. To see this, trying to write the triples corresponding to what is shown in Figure 6 would produce something like:

exstaff:85740   exterms:address      ??? .
???             exterms:street       "1501 Grant Avenue" .
???             exterms:city         "Bedford" .
???             exterms:state        "Massachusetts" .
???             exterms:postalCode   "01730" .

where ??? stands for something that indicates the presence of the blank node. Since a complex graph might contain more than one blank node, there also needs to be a way to differentiate between these different blank nodes in a triples representation of the graph. As a result, triples use blank node identifiers, having the form _:name, to indicate the presence of blank nodes. For instance, in this example a blank node identifier _:johnaddress might be used to refer to the blank node, in which case the resulting triples might be:

exstaff:85740   exterms:address     _:johnaddress .
_:johnaddress   exterms:street      "1501 Grant Avenue" .
_:johnaddress   exterms:city        "Bedford" .
_:johnaddress   exterms:state       "Massachusetts" .
_:johnaddress   exterms:postalCode  "01730" .

In a triples representation of a graph, each distinct blank node in the graph is given a different blank node identifier. Unlike URIrefs and literals, blank node identifiers are not considered to be actual parts of the RDF graph (this can be seen by looking at the drawn graph in Figure 6 and noting that the blank node has no blank node identifier). Blank node identifiers are just a way of representing the blank nodes in a graph (and distinguishing one blank node from another) when the graph is written in triple form. Blank node identifiers also have significance only within the triples representing a single graph (two different graphs with the same number of blank nodes might independently use the same blank node identifiers to distinguish them, and it would be incorrect to assume that blank nodes from different graphs having the same blank node identifiers are the same). If it is expected that a node in a graph will need to be referenced from outside the graph, a URIref should be assigned to identify it. Finally, because blank node identifiers represent (blank) nodes, rather than arcs, in the triple form of an RDF graph, blank node identifiers may only appear as subjects or objects in triples; blank node identifiers may not be used as predicates in triples.

The beginning of this section noted that aggregate structures, like John Smith's address, can be represented by considering the aggregate thing to be described as a separate resource, and then making statements about that new resource. This example illustrates an important aspect of RDF: RDF directly represents only binary relationships, e.g. the relationship between John Smith and the literal representing his address. Representing the relationship between John and the group of separate components of this address involves dealing with an n-ary (n-way) relationship (in this case, n=5) between John and the street, city, state, and postal code components. In order to represent such structures directly in RDF (e.g., considering the address as a group of street, city, state, and postal code components), this n-way relationship must be broken up into a group of separate binary relationships. Blank nodes provide one way to do this. For each n-ary relationship, one of the participants is chosen as the subject of the relationship (John in this case), and a blank node is created to represent the rest of the relationship (John's address in this case). The remaining participants in the relationship (such as the city in this example) are then represented as separate properties of the new resource represented by the blank node.

Blank nodes also provide a way to more accurately make statements about resources that may not have URIs, but that are described in terms of relationships with other resources that do have URIs. For example, when making statements about a person, say Jane Smith, it may seem natural to use a URI based on that person's email address as her URI, e.g., mailto:jane@example.org. However, this approach can cause problems. For example, it may be necessary to record information both about Jane's mailbox (e.g., the server it is on) as well as about Jane herself (e.g., her current physical address), and using a URIref for Jane based on her email address makes it difficult to know whether it is Jane or her mailbox that is being described. The same problem exists when a company's Web page URL, say http://www.example.com/, is used as the URI of the company itself. Once again, it may be necessary to record information about the Web page itself (e.g., who created it and when) as well as about the company, and using http://www.example.com/ as an identifier for both makes it difficult to know which of these is the actual subject.

The fundamental problem is that using Jane's mailbox as a stand-in for Jane is not really accurate: Jane and her mailbox are not the same thing, and hence they should be identified differently. When Jane herself does not have a URI, a blank node provides a more accurate way of modeling this situation. Jane can be represented by a blank node, and that blank node used as the subject of a statement with exterms:mailbox as the property and the URIref mailto:jane@example.org as its value. The blank node could also be described with an rdf:type property having a value of exterms:Person (types are discussed in more detail in the following sections), an exterms:name property having a value of "Jane Smith", and any other descriptive information that might be useful, as shown in the following triples:

_:jane   exterms:mailbox   <mailto:jane@example.org> .
_:jane   rdf:type          exterms:Person .
_:jane   exterms:name      "Jane Smith" .
_:jane   exterms:empID     "23748"  .
_:jane   exterms:age       "26" .

(Note that mailto:jane@example.org is written within angle brackets in the first triple. This is because mailto:jane@example.org is a full URIref in the mailto URI scheme, rather than a QName abbreviation, and full URIrefs must be enclosed in angle brackets in the triples notation.)

This says, accurately, that "there is a resource of type exterms:Person, whose electronic mailbox is identified by mailto:jane@example.org, whose name is Jane Smith, etc." That is, the blank node can be read as "there is a resource". Statements with that blank node as subject then provide information about the characteristics of that resource.

In practice, using blank nodes instead of URIrefs in these cases does not change the way this kind of information is handled very much. For example, if it is known that an email address uniquely identifies someone at example.org (particularly if the address is unlikely to be reused), that fact can still be used to associate information about that person from multiple sources, even though the email address is not the person's URI. In this case, if some RDF is found on the Web that describes a book, and gives the author's contact information as mailto:jane@example.org, it might be reasonable, combining this new information with the previous set of triples, to conclude that the author's name is Jane Smith. The point is that saying something like "the author of the book is mailto:jane@example.org" is typically a shorthand for "the author of the book is someone whose mailbox is mailto:jane@example.org". Using a blank node to represent this "someone" is just a more accurate way to represent the real world situation. (Incidentally, some RDF-based schema languages allow specifying that certain properties are unique identifiers of the resources they describe. This is discussed further in Section 5.5.)

Using blank nodes in this way can also help avoid the use of literals in what might be inappropriate situations. For example, in describing Jane's book, lacking a URIref to identify the author, the publisher might have written (using the publisher's own ex2terms: vocabulary):

ex2terms:book78354   rdf:type          ex2terms:Book .
ex2terms:book78354   ex2terms:author   "Jane Smith" .

However, the author of the book is not really the character string "Jane Smith", but a person whose name is Jane Smith. The same information might be more accurately given by the publisher using a blank node, as:

ex2terms:book78354   rdf:type         ex2terms:Book .
ex2terms:book78354   ex2terms:author  _:author78354 .
_:author78354        rdf:type         ex2terms:Person .
_:author78354        ex2terms:name    "Jane Smith" .

This essentially says "resource ex2terms:book78354 is of type ex2terms:Book, and its author is a resource of type ex2terms:Person, whose name is Jane Smith." Of course, in this particular case the publisher might instead have assigned its own URIrefs to its authors instead of using blank nodes to identify them, in order to encourage external references to its authors.

Finally, the example above giving Jane's age as 26 illustrates the fact that sometimes the value of a property may appear to be simple, but actually may be more complex. In this case, Jane's age is actually 26 years, but the units information (years) is not explicitly given. Such information is often omitted in contexts where it can be safely assumed that anyone accessing the property value will understand the units being used. However, in the wider context of the Web, it is generally not safe to make this assumption. For example, a U.S. site might give a weight value in pounds, but someone accessing that data from outside the U.S. might assume that weights are given in kilograms. In general, careful consideration should be given to explicitly representing units and similar information. This issue is discussed further in Section 4.4, which describes an RDF feature for representing such information as structured values, as well as some other techniques for representing such information.

2.4 Typed Literals

The last section described how to handle situations in which property values represented by plain literals had to be broken up into structured values to represent the individual parts of those literals. Using this approach, instead of, say, recording the date a Web page was created as a single exterms:creation-date property, with a single plain literal as its value, the value would be represented as a structure consisting of the month, day, and year as separate pieces of information, using separate plain literals to represent the corresponding values. However, so far, all constant values that serve as objects in RDF statements have been represented by these plain (untyped) literals, even when the intent is probably for the value of the property to be a number (e.g., the value of a year or age property) or some other kind of more specialized value.

For example, Figure 4 illustrated an RDF graph recording information about John Smith. That graph recorded the value of John Smith's exterms:age property as the plain literal "27", as shown in Figure 7:

In this case, the hypothetical organization example.org probably intends for "27" to be interpreted as a number, rather than as the string consisting of the character "2" followed by the character "7" (since the literal represents the value of an "age" property). However, there is no information in Figure 7's graph that explicitly indicates that "27" should be interpreted as a number. Similarly, example.org also probably intends for "27" to be interpreted as a decimal number, i.e., the value twenty seven, rather than, say, as an octal number, i.e., the value twenty three. However, once again there is no information in Figure 7's graph that explicitly indicates this. Specific applications might be written with the understanding that they should interpret values of the exterms:age property as decimal numbers, but this would mean that proper interpretation of this RDF would depend on information not explicitly provided in the RDF graph, and hence on information that would not necessarily be available to other applications that might need to interpret this RDF.

The common practice in programming languages or database systems is to provide this additional information about how to interpret a literal by associating a datatype with the literal, in this case, a datatype like decimal or integer. An application that understands the datatype then knows, for example, whether the literal "10" is intended to represent the number ten, the number two, or the string consisting of the character "1" followed by the character "0", depending on whether the specified datatype is integer, binary, or string. (More specialized datatypes could also be used to include the units information mentioned at the end of Section 2.3, e.g., a datatype integerYears, although the Primer will not elaborate on this idea.) In RDF, typed literals are used to provide this kind of information.

An RDF typed literal is formed by pairing a string with a URIref that identifies a particular datatype. This results in a single literal node in the RDF graph with the pair as the literal. The value represented by the typed literal is the value that the specified datatype associates with the specified string. For example, using a typed literal, John Smith's age could be described as being the integer number 27 using the triple:

<http://www.example.org/staffid/85740>  <http://www.example.org/terms/age> "27"^^<http://www.w3.org/2001/XMLSchema#integer> .

or, using the QName simplification for writing long URIs:

exstaff:85740  exterms:age  "27"^^xsd:integer .

or as shown in Figure 8:

Similarly, in the graph shown in Figure 3 describing information about a Web page, the value of the page's exterms:creation-date property was written as the plain literal "August 16, 1999". However, using a typed literal, the creation date of the Web page could be explicitly described as being the date August 16, 1999, using the triple:

ex:index.html  exterms:creation-date  "1999-08-16"^^xsd:date .

or as shown in Figure 9:

Unlike typical programming languages and database systems, RDF has no built-in set of datatypes of its own, such as datatypes for integers, reals, strings, or dates. Instead, RDF typed literals simply provide a way to explicitly indicate, for a given literal, what datatype should be used to interpret it. The datatypes used in typed literals are defined externally to RDF, and identified by their datatype URIs. (There is one exception: RDF defines a built-in datatype with the URIref rdf:XMLLiteral to represent XML content as a literal value. This datatype is defined in [RDF-CONCEPTS], and its use is described in Section 4.5.) For instance, the examples in Figure 8 and Figure 9 use the datatypes integer and date from the XML Schema datatypes defined in XML Schema Part 2: Datatypes [XML-SCHEMA2]. An advantage of this approach is that it gives RDF the flexibility to directly represent information coming from different sources without the need to perform type conversions between these sources and a native set of RDF datatypes. (Type conversions would still be required when moving information between systems having different sets of datatypes, but RDF would impose no extra conversions into and out of a native set of RDF datatypes.)

RDF datatype concepts are based on a conceptual framework from XML Schema datatypes [XML-SCHEMA2], as described in RDF Concepts and Abstract Syntax [RDF-CONCEPTS]. This conceptual framework defines a datatype as consisting of:

  • A set of values, called the value space, that literals of the datatype are intended to represent. For example, for the XML Schema datatype xsd:date, this set of values is a set of dates.
  • A set of character strings, called the lexical space, that the datatype uses to represent its values. This set determines which character strings can legally be used to represent literals of this datatype. For example, the datatype xsd:date defines 1999-08-16 as being a legal way to write a literal of this type (as opposed, say, to August 16, 1999). As defined in [RDF-CONCEPTS], the lexical space of a datatype is a set of Unicode [UNICODE] strings, allowing information from many languages to be directly represented.
  • A lexical-to-value mapping from the lexical space to the value space. This determines the value that a given character string from the lexical space represents for this particular datatype. For example, the lexical-to-value mapping for datatype xsd:date determines that, for this datatype, the string 1999-08-16 represents the date August 16, 1999. The lexical-to-value mapping is a factor because the same character string may represent different values for different datatypes.

Not all datatypes are suitable for use in RDF. For a datatype to be suitable for use in RDF, it must conform to the conceptual framework just described. This basically means that, given a character string, the datatype must unambiguously define whether or not the string is in its lexical space, and what value in its value space the string represents. For example, the basic XML Schema datatypes such as xsd:string, xsd:boolean, xsd:date, etc. are suitable for use in RDF. However, some of the built-in XML Schema datatypes are not suitable for use in RDF. For example, xsd:duration does not have a well-defined value space, and xsd:QName requires an enclosing XML document context. Lists of the XML Schema datatypes that are currently considered suitable and unsuitable for use in RDF are given in [RDF-SEMANTICS].

Since the value that a given typed literal denotes is defined by the typed literal's datatype, and, with the exception of rdf:XMLLiteral, RDF does not define any datatypes, the actual interpretation of a typed literal appearing in an RDF graph (e.g., determining the value it denotes) must be performed by software that is written to correctly process not only RDF, but the typed literal's datatype as well. Effectively, this software must be written to process an extended language that includes not only RDF, but also the datatype, as part of its built-in vocabulary. This raises the issue of which datatypes will be generally available in RDF software. Generally, the XML Schema datatypes that are listed as suitable for use in RDF in [RDF-SEMANTICS] have a "first among equals" status in RDF. As noted already, the examples in Figure 8 and Figure 9 used some of these XML Schema datatypes, and the Primer will be using these datatypes in most of its other examples of typed literals as well (for one thing, XML Schema datatypes already have assigned URIrefs that can be used to refer to them, specified in [XML-SCHEMA2]). These XML Schema datatypes are treated no differently than any other datatype, but they are expected to be the most widely used, and therefore the most likely to be interoperable among different software. As a result, it is expected that much RDF software will also be written to process these datatypes. However, RDF software could be written to process other sets of datatypes as well, assuming they were determined to be suitable for use with RDF, as described already.

In general, RDF software may be called on to process RDF data that contains references to datatypes that the software has not been written to process, in which case there are some things the software will not be able to do. For one thing, with the exception of rdf:XMLLiteral, RDF itself does not define the URIrefs that identify datatypes. As a result, RDF software, unless it has been written to recognize specific URIrefs, will not be able to determine whether or not a URIref written in a typed literal actually identifies a datatype. Moreover, even when a URIref does identify a datatype, RDF itself does not define the validity of pairing that datatype with a particular literal. This validity can only be determined by software written to correctly process that particular datatype.

For example, the typed literal in the triple:

exstaff:85740  exterms:age  "pumpkin"^^xsd:integer .

or the graph shown in Figure 10:

is valid RDF, but obviously an error as far as the xsd:integer datatype is concerned, since "pumpkin" is not defined as being in the lexical space of xsd:integer. RDF software not written to process the xsd:integer datatype would not be able to recognize this error.

However, proper use of RDF typed literals provides more information about the intended interpretation of literal values, and hence makes RDF statements a better means of information exchange among applications.

2.5 Concepts Summary

Taken as a whole, RDF is basically simple: nodes-and-arcs diagrams interpreted as statements about things identified by URIrefs. This section has presented an introduction to these concepts. As noted earlier, the normative (i.e., definitive) RDF specification describing these concepts is RDF Concepts and Abstract Syntax [RDF-CONCEPTS], which should be consulted for further information. The formal semantics (meaning) of these concepts is defined in the (normative) RDF Semantics [RDF-SEMANTICS] document.

However, in addition to the basic techniques for describing things using RDF statements discussed so far, it should be clear that people or organizations also need a way to describe the vocabularies (terms) they intend to use in those statements, specifically, vocabularies for:

The basis for describing such vocabularies in RDF is the RDF Vocabulary Description Language 1.0: RDF Schema [RDF-VOCABULARY], which will be described in Section 5.

Additional background on the basic ideas underlying RDF, and its role in providing a general language for describing Web information, can be found in [WEBDATA]. RDF draws upon ideas from knowledge representation, artificial intelligence, and data management, including Conceptual Graphs, logic-based knowledge representation, frames, and relational databases. Some possible sources of background information on these subjects include [SOWA], [CG], [KIF], [HAYES], [LUGER], and [GRAY].

3. A textual syntax for RDF: Turtle

As described in Section 2, RDF's conceptual model is a graph. Turtle provides an textual syntax for writing down and exchanging RDF graphs. This syntax includes the triple notation used in this document so far, but also includes some additional syntactic means to simplify the specification of RDF graphs. Turtle is defined in the @@@Final Title of the document@@@ [TURTLE]. This section describes this syntax.

3.1 Basic Principles

The basic ideas behind the Turtle syntax can be illustrated using some of the examples presented already. Take as an example the English statement:

http://www.example.org/index.html has a creation-date whose value is August 16, 1999

The RDF graph for this single statement, after assigning a URIref to the creation-date property, is shown in Figure 11:

with a triple representation of:

ex:index.html   exterms:creation-date   "August 16, 1999" .

(Note that a typed literal is not used for the date value in this example. Representing typed literals in Turtle will be described later in this section.)

Example 2 shows the Turtle syntax corresponding to the graph in Figure 11:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix exterms: <http://www.example.org/terms/>.

<http://www.example.org/index.html> exterms:creation-date "August 16, 1999".

The only additional syntactic element to what has been used so far is the usage of the @prefix keyword in first two lines. These are the namespace declarations, ie, the association of a prefix used in the document with a URI; one line per prefix.

The Turtle syntax provides a number of abbreviations to make common uses easier to write. For example, it is typical for the same resource to be described with several properties and values at the same time. To handle such cases, Turtle allows multiple property elements representing those properties to be nested within the symbol element that identifies the subject resource. For example, to represent the following group of statements about http://www.example.org/index.html:

ex:index.html   dc:creator              exstaff:85740 .
ex:index.html   exterms:creation-date   "August 16, 1999" .
ex:index.html   dc:language             "en" .

whose graph (the same as Figure 3) is shown in Figure 12:

the Turtle version of the graph shown in Figure 12 could be written as:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix dc:      <http://purl.org/dc/elements/1.1/#>.
@prefix exterms: <http://www.example.org/terms/>.

    exterms:creation-date "August 16, 1999";
    dc:language "en";
    dc:creator <http://www.example.org/staffid/85740>.

Compared with the previous examples, Example 3 adds an additional dc:language dc:creator properties to the same subject. Note the usage of the ";" character: this character separates the additional property-object pairs for the same subject. The series of such pairs is closed with the "." character. Note also that whitespace characters can be used anywhere to ensure a more readable format.

It is important to understand that Example 3 is an abbreviation. Example 4, in which each statement is written separately, describes exactly the same RDF graph (the graph of Figure 12):

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix dc:      <http://purl.org/dc/elements/1.1/#>.
@prefix exterms: <hhttp://www.example.org/terms/>.

<http://www.example.org/index.html> exterms:creation-date "August 16, 1999".
<http://www.example.org/index.html> dc:language           "en".
<http://www.example.org/index.html> dc:creator            <http://www.example.org/staffid/85740>.

The following sections will describe a few additional Turtle abbreviations. The Turtle specification document[TURTLE] provides a more thorough description of the abbreviations that are available.

Turtle can also represent graphs that include nodes that have no URIrefs, i.e., the blank nodes described in Section 2.3. For example, Figure 13 (taken from [RDF-SYNTAX]) shows a graph saying "the document 'http://www.w3.org/TR/rdf-syntax-grammar' has a title 'RDF/XML Syntax Specification (Revised)' and has an editor, the editor has a name 'Dave Beckett' and a home page 'http://purl.org/net/dajobe/' ".

This illustrates an idea discussed in Section 2.3: the use of a blank node to represent something that does not have a URIref, but can be described in terms of other information. In this case, the blank node represents a person, the editor of the document, and the person is described by his name and home page.

Turtle provides several ways to represent graphs containing blank nodes. The approach illustrated here, which is the most direct approach, is to assign a blank node identifier to each blank node (just as for direct triples in our previous examples). A blank node identifier serves to identify a blank node within a particular Turtle document but, unlike a URIref, is unknown outside the document in which it is assigned. A blank node is referred to in Turtle using the special namespace prefix "_" with a blank node identifier as its value, in places where the URIref of a resource would otherwise appear. Using this facility, Example 5 shows the Turtle corresponding to Figure 13:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix dc:      <http://purl.org/dc/elements/1.1/#>.
@prefix exterms: <http://www.example.org/terms/>.

    dc:title "RDF/XML Syntax Specification (Revised)";
    exterm:editor _:abc.
    exterms:fullName "Dave Beckett";
    exterms:homePage <http://purl.org/net/dajobe/>.

In Example 5, the blank node identifier abc is used to identify the blank node as the subject of several statements, and is used separately to indicate that the blank node is the value of a resource's exterms:editor property. The advantage of using a blank node identifier is that using a blank node identifier allows the same blank node to be referred to in more than one place in the same Turtle document.

However, the reuse of the blank node from within the Turtle document is not always necessary. Turtle also provides an abbreviated syntax using anonymous blank nodes, ie, without the necessity to define local names. Example 6 shows another approach of Turtle for the serialization of the graph on Figure 13:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix dc:      <http://purl.org/dc/elements/1.1/#>.
@prefix exterms: <http://www.example.org/terms/>.

    dc:title "RDF/XML Syntax Specification (Revised)";
    exterm:editor [
        exterms:fullName "Dave Beckett";
        exterms:homePage <http://purl.org/net/dajobe/>

In Example 6, the pair of [ and ] enclose a blank node, whose identification is left to the system (essentially the parser). Although this approach does not give the possibility to access this blank node from other parts of the same Turtle file, it covers a large number of cases of blank node usage.

In the rest of the Primer, the examples will use typed literals from appropriate datatypes rather than plain (untyped) literals, in order to emphasize the value of typed literals in conveying more information about the intended interpretation of literal values. (The exceptions will be that plain literals will continue to be used in examples taken from actual applications that do not currently use typed literals, in order to accurately reflect the usage in those applications.) In Turtle, both plain and typed literals (and, with certain exceptions, tags) can contain Unicode [UNICODE] characters, allowing information from many languages to be directly represented.

3.2 Abbreviating and Organizing RDF URIrefs

So far, the examples have assumed that the resources being described have been given URIrefs already. For instance, the initial examples provided descriptive information about example.org's Web page, whose URIref was http://www.example.org/index.html. This resource was identified in Turtle using its full URIref. Although RDF does not specify or control how URIrefs are assigned to resources, sometimes it is desirable to achieve the effect of assigning URIrefs to resources that are part of an organized group of resources. For example, suppose a sporting goods company, example.com, wanted to provide an RDF-based catalog of its products, such as tents, hiking boots, and so on, as an RDF/XML document, identified by (and located at) http://www.example.com/2002/04/products. In that resource, each product might be given a separate RDF description. This catalog, along with one of these descriptions, the catalog entry for a model of tent called the "Overnighter", might be written in Turtle as shown in Example 7:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix xsd:     <http://www.w3.org/2001/XMLSchema#>.
@prefix exterms: <hhttp://www.example.org/terms/>.

   exterm:models     "Overnighter"^^xsd:string;
   exterm:sleeps     "2"^^xsd:integer;
   exterm:weight     "2.4"^^xsd:decimal;
   exterm:packedSize "784"^^xsd:integer.

  ...other product descriptions...

Example 7 is similar to previous examples in the way it represents the properties (model, sleeping capacity, weight) of the resource (the tent) being described. Note also that although the datatypes associated with the various property values are given explicitly, the units associated with some of these property values are not, even though this information should be available to properly interpret the values. Representing units and similar information that may be associated with property values is discussed in Section 4.4. In this example, the value of exterms:sleeps is the number of persons the tent can sleep, the value of exterms:weight is given in kilograms, and the value of exterms:packedSize is given in square centimeters, the area the tent occupies on a backpack.)

An important difference from previous examples is the usage of the :item10245 to identify a resource. Using a namespace without a prefix specifies a fragment identifier, as an abbreviation of the complete URIref of the resource being described. The fragment identifier item10245 will be interpreted relative to a base URI, in this case the URI of the containing catalog document. The full URIref for the tent is formed by taking the base URI (of the catalog), and appending the character "#" (to indicate that what follows is a fragment identifier) and then item10245 to it, giving the absolute URIref http://www.example.com/2002/04/products#item10245.

This formalism is somewhat similar to the id attribute in XML and HTML, in that it defines a name which must be unique relative to the current base URI (in this example, that of the catalog). Any other Turtle statement within this catalog could refer to the tent by using either the absolute URIref http://www.example.com/2002/04/products#item10245, or the relative URIref :item10245. The relative URIref would be understood as being a URIref defined relative to the base URIref of the catalog. This use of a relative URIref can be thought of either as being an abbreviation for a full URIref that has been assigned to the tent independently of the RDF, or as being the assignment of the URIref to the tent within the catalog.

RDF located outside the catalog could refer to this tent by using the full URIref, i.e., by concatenating the relative URIref #item10245 of the tent to the base URI of the catalog, forming the absolute URIref http://www.example.com/2002/04/products#item10245. For example, an outdoor sports Web site exampleRatings.com might use RDF to provide ratings of various tents. The (5-star) rating given to the tent described in Example 7 might then be represented on exampleRatings.com's Web site as shown in Example 8:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix sportex: <http://www.exampleRatings.com/terms/>.
@prefix xsd:     <http://www.w3.org/2001/XMLSchema#>.

     sportex:ratingBy    "Richard Roe"^^xsd:string;
     sportex:numberStars "5"^^xsd:integer.

Example 8 uses a full URIref of the tent. The use of this URIref allows the tent being referred to in the rating to be precisely identified.

These examples illustrate several points. First, even though RDF does not specify or control how URIrefs are assigned to resources (in this case, the various tents and other items in the catalog), the effect of assigning URIrefs to resources in RDF can be achieved by combining a process (external to RDF) that identifies a single document (the catalog in this case) as the source for descriptions of those resources, with the use of relative URIrefs in descriptions of those resources within that document. For instance, example.com could use this catalog as the central source where its products are described, with the understanding that if a product's item number is not in an entry in this catalog, it is not a product known to example.com. (Note that RDF does not assume any particular relationship exists between two resources just because their URIrefs have the same base, or are otherwise similar. This relationship may be known to example.com, but it is not directly defined by RDF.)

These examples also illustrate one of the basic architectural principles of the Web, which is that anyone should be able to freely add information about an existing resource, using any vocabulary they please [BERNERS-LEE98]. The examples further illustrate that the RDF describing a particular resource does not need to be located all in one place; instead, it may be distributed throughout the Web. This is true not only for situations like this one, in which one organization is rating or commenting on a resource defined by another, but also for situations in which the original definer of a resource (or anyone else) wishes to amplify the description of that resource by providing additional information about it. This may be done by modifying the RDF document in which the resource was originally described, to add the properties and values needed to describe the additional information. Or, as this example illustrates, a separate document could be created, providing the additional properties and values, referring to the original resource via its URIref.

The discussion above indicated that relative URIrefs such as :item10245 will be interpreted relative to a base URI. By default, this base URI would be the URI of the resource in which the relative URIref is used. However, in some cases it is desirable to be able to explicitly specify this base URI. For instance, suppose that in addition to the catalog located at http://www.example.com/2002/04/products, example.org wanted to provide a duplicate catalog on a mirror site, say at http://mirror.example.com/2002/04/products. This could create a problem, since if the catalog was accessed from the mirror site, the URIref for the example tent would be generated from the URI of the containing document, forming http://mirror.example.com/2002/04/products#item10245, rather than http://www.example.com/2002/04/products#item10245, and hence would apparently refer to a different resource than the one intended. Alternatively, example.org might want to assign a base URIref for its set of product URIrefs without publishing a single source document whose location defines the base.

To deal with such cases, Turtle support a @base facility, which allows a Turtle document to specify a base URI other than the URI of the document itself. Example 9 shows how the catalog would be described using XML Base:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix exterms: <hhttp://www.example.org/terms/>.
@prefix xsd:     <http://www.w3.org/2001/XMLSchema#>.
@base            <http://www.example.com/2002/04/products>.

   exterm:models     "Overnighter"^^xsd:string;
   exterm:sleeps     "2"^^xsd:integer;
   exterm:weight     "2.4"^^xsd:decimal;
   exterm:packedSize "784"^^xsd:integer.

  ...other product descriptions...

In Example 10, the @base declaration specifies that the base URI for the content within the file element is http://www.example.com/2002/04/products, and all relative URIrefs cited within that content will be interpreted relative to that base, no matter what the URI of the containing document is. As a result, the relative URIref of the tent, #item10245, will be interpreted as the same absolute URIref, http://www.example.com/2002/04/products#item10245, no matter what the actual URI of the catalog document is, or whether the base URIref actually identifies a particular document at all.

So far, the examples have used a single product description, a particular model of tent, from example.com's catalog. However, example.com will probably offer several different models of tents, as well as multiple instances of other categories of products, such as backpacks, hiking boots, and so on. This idea of things being classified into different kinds or categories is similar to the programming language concept of objects having different types or classes. RDF supports this concept by providing a predefined property, rdf:type. When an RDF resource is described with an rdf:type property, the value of that property is considered to be a resource that represents a category or class of things, and the subject of that property is considered to be an instance of that category or class. Using rdf:type, Example 10 shows how example.com might indicate that the product description is that of a tent:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix exterms: <http://www.example.org/terms/>.
@prefix xsd:     <http://www.w3.org/2001/XMLSchema#>.
@base <http://www.example.com/2002/04/products>.

   rdf:type          <http://www.example.com/terms/Tent>;
   exterm:models     "Overnighter"^^xsd:string;
   exterm:sleeps     "2"^^xsd:integer;
   exterm:weight     "2.4"^^xsd:decimal;
   exterm:packedSize "784"^^xsd:integer.

In Example 10, the rdf:type property indicates that the resource being described is an instance of the class identified by the URIref http://www.example.com/terms/Tent. This assumes that example.com has described its classes as part of the same vocabulary that it uses to describe its other terms (such as the property exterms:weight), so the absolute URIref of the class is used to refer to it. If example.com had described these classes as part of the product catalog itself, the relative URIref :Tent could have been used to refer to it.

RDF itself does not provide facilities for defining application-specific classes of things, such as Tent in this example, or their properties, such as exterms:weight. Instead, such classes would be described in an RDF schema, using the RDF Schema language discussed in Section 5. Other such facilities for describing classes can also be defined, such as the DAML+OIL and OWL languages described in Section 5.5.

It is fairly common in RDF for resources to have rdf:type properties that describe the resources as instances of specific types or classes. Turtle introduces a special abbreviation for the rdf:type property using the a keyword. Using this abbreviation, example.com's tent from Example 10 could also be described as shown in Example 11:

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix exterms: <hhttp://www.example.org/terms/>.
@prefix xsd:     <http://www.w3.org/2001/XMLSchema#>.
@base <http://www.example.com/2002/04/products>.

:item10245 a <http://www.example.com/terms/Tent>;
   exterm:models     "Overnighter"^^xsd:string;
   exterm:sleeps     "2"^^xsd:integer;
   exterm:weight     "2.4"^^xsd:decimal;
   exterm:packedSize "784"^^xsd:integer.

4. Other RDF Capabilities

RDF provides a number of additional capabilities, such as built-in types and properties for representing groups of resources and RDF statements, and capabilities for representing XML fragments as property values. These additional capabilities are described in the following sections.

4.1 RDF Containers

There is often a need to describe groups of things: for example, to say that a book was created by several authors, or to list the students in a course, or the software modules in a package. RDF provides several predefined (built-in) types and properties that can be used to describe such groups.

First, RDF provides a container vocabulary consisting of three predefined types (together with some associated predefined properties). A container is a resource that contains things. The contained things are called members. The members of a container may be resources (including blank nodes) or literals. RDF defines three types of containers:

A Bag (a resource having type rdf:Bag) represents a group of resources or literals, possibly including duplicate members, where there is no significance in the order of the members. For example, a Bag might be used to describe a group of part numbers in which the order of entry or processing of the part numbers does not matter.

A Sequence or Seq (a resource having type rdf:Seq) represents a group of resources or literals, possibly including duplicate members, where the order of the members is significant. For example, a Sequence might be used to describe a group that must be maintained in alphabetical order.

An Alternative or Alt (a resource having type rdf:Alt) represents a group of resources or literals that are alternatives (typically for a single value of a property). For example, an Alt might be used to describe alternative language translations for the title of a book, or to describe a list of alternative Internet sites at which a resource might be found. An application using a property whose value is an Alt container should be aware that it can choose any one of the members of the group as appropriate.

To describe a resource as being one of these types of containers, the resource is given an rdf:type property whose value is one of the predefined resources rdf:Bag, rdf:Seq, or rdf:Alt (whichever is appropriate). The container resource (which may either be a blank node or a resource with a URIref) denotes the group as a whole. The members of the container can be described by defining a container membership property for each member with the container resource as its subject and the member as its object. These container membership properties have names of the form rdf:_n , where n is a decimal integer greater than zero, with no leading zeros, e.g., rdf:_1, rdf:_2, rdf:_3, and so on, and are used specifically for describing the members of containers. Container resources may also have other properties that describe the container, in addition to the container membership properties and the rdf:type property.

It is important to understand that while these types of containers are described using predefined RDF types and properties, any special meanings associated with these containers, e.g., that the members of an Alt container are alternative values, are only intended meanings. These specific container types, and their definitions, are provided with the aim of establishing a shared convention among those who need to describe groups of things. All RDF does is provide the types and properties that can be used to construct the RDF graphs to describe each type of container. RDF has no more built-in understanding of what a resource of type rdf:Bag is than it has of what a resource of type ex:Tent (discussed in Section 3.2) is. In each case, applications must be written to behave according to the particular meaning involved for each type. This point will be expanded on in the following examples.

A typical use of a container is to indicate that the value of a property is a group of things. For example, to represent the sentence "Course 6.001 has the students Amy, Mohamed, Johann, Maria, and Phuong", the course could be described by giving it a s:students property (from an appropriate vocabulary) whose value is a container of type rdf:Bag (representing the group of students). Then, using the container membership properties, individual students could be identified as being members of that group, as in the RDF graph shown in Figure 14:

Since the value of the s:students property in this example is described as a Bag, there is no intended significance in the order given for the URIrefs of the students, even though the membership properties in the graph have integers in their names. It is up to applications creating and processing graphs that include rdf:Bag containers to ignore any (apparent) order in the names of the membership properties.

Example 12 describes the graph shown in Figure 14:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix s:   <http://example.org/students/vocab#>.

    s:students [
        a rdf:Bag;
        rdf:_1 <http://example.org/students/Amy>;
        rdf:_2 <http://example.org/students/Mohamed>;
        rdf:_3 <http://example.org/students/Johann>;
        rdf:_4 <http://example.org/students/Maria>;
        rdf:_5 <http://example.org/students/Phuong>.

The graph structure for an rdf:Seq container, and the corresponding Turtle, are similar to those for an rdf:Bag (the only difference is in the type, rdf:Seq). Once again, although an rdf:Seq container is intended to describe a sequence, it is up to applications creating and processing the graph to appropriately interpret the sequence of integer-valued property names.

Note that this example also shows a Turtle abbreviation for the property http://www.w3.org/1999/02/22-rdf-syntax-ns#type in the form of the special predicate name "a". Because using rdf typing is very frequent, this abbreviation is very useful in practice.

To illustrate an Alt container, the sentence "The source code for X11 may be found at ftp.example.org, ftp1.example.org, or ftp2.example.org" could be expressed in the RDF graph shown in Figure 15:

Example 13 shows how the graph in Figure 15 could be written in Turtle:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix s:   <http://example.org/packages/vocab#>.

    s:DistributionSite [
        a rdf:Alt;
        rdf:_1 <ftp://ftp.example.org>;
        rdf:_2 <ftp://ftp1.example.org>;
        rdf:_3 <ftp://ftp2.example.org>.

An Alt container is intended to have at least one member, identified by the property rdf:_1. This member is intended to be considered as the default or preferred value. Other than the member identified as rdf:_1, the order of the remaining elements is not significant.

The RDF in Figure 15 as written states simply that the value of the s:DistributionSite site property is the Alt container resource itself. Any additional meaning that is to be read into this graph, e.g., that one of the members of the Alt container is to be considered as the value of the s:DistributionSite site property, or that ftp://ftp.example.org is the default or preferred value, must be built into an application's understanding of the intended meaning of an Alt container, and/or into the meaning defined for the particular property (s:DistributionSite in this case), which also must be understood by the application.

Alt containers are frequently used in conjunction with language tagging. (Turtle permits the use of a language tag to indicate that the element content is in a specified language. The use of this tag is described in [TURTLE], and illustrated later in Section 6.2.) For example, a work whose title has been translated into several languages might have its title property pointing to an Alt container holding literals representing the titles expressed in each of the language variants.

The distinction between the intended meanings of a Bag and an Alt can be further illustrated by considering the authorship of the book "Huckleberry Finn". The book has exactly one author, but the author has two names (Mark Twain and Samuel Clemens). Either name is sufficient to specify the author. Thus using an Alt container for the author's names more accurately represents the relationship than using a Bag (which might suggest there are two different authors).

Users are free to choose their own ways to describe groups of resources, rather than using the RDF container vocabulary. These RDF containers are merely provided as common definitions that, if generally used, could help make data involving groups of resources more interoperable.

Sometimes there are clear alternatives to using these RDF container types. For example, a relationship between a particular resource and a group of other resources could be indicated by making the first resource the subject of multiple statements using the same property. This is structurally different from the resource being the subject of a single statement whose object is a container containing multiple members. In some cases, these two structures may have equivalent meaning, but in other cases they may not. The choice of which to use in a given situation should be made with this in mind.

Consider as an example the relationship between a writer and her publications, as in the sentence:

Sue has written "Anthology of Time", "Zoological Reasoning", and "Gravitational Reflections".

In this case, there are three resources each of which was written independently by the same writer. This could be expressed using repeated properties as:

exstaff:Sue   exterms:publication   ex:AnthologyOfTime .
exstaff:Sue   exterms:publication   ex:ZoologicalReasoning .
exstaff:Sue   exterms:publication   ex:GravitationalReflections .

In this example there is no stated relationship between the publications other than that they were written by the same person. Each of the statements is an independent fact, and so using repeated properties would be a reasonable choice. However, this could just as reasonably be represented as a statement about the group of resources written by Sue:

exstaff:Sue   exterms:publication _:z
_:z           rdf:type            rdf:Bag .
_:z           rdf:_1              ex:AnthologyOfTime .
_:z           rdf:_2              ex:ZoologicalReasoning .
_:z           rdf:_3              ex:GravitationalReflections .

On the other hand, the sentence:

The resolution was approved by the Rules Committee, having members Fred, Wilma, and Dino.

says that the committee as a whole approved the resolution; it does not necessarily state that each committee member individually voted in favor of the resolution. In this case, it would be potentially misleading to model this sentence as three separate exterms:approvedBy statements, one for each committee member, as shown below:

ex:resolution   exterms:approvedBy   ex:Fred .
ex:resolution   exterms:approvedBy   ex:Wilma .
ex:resolution   exterms:approvedBy   ex:Dino .

since these statements say that each member individually approved the resolution.

In this case, it would be better to model the sentence as a single exterms:approvedBy statement whose subject is the resolution and whose object is the committee itself. The committee resource could then be described as a Bag whose members are the members of the committee, as in the following triples:

ex:resolution      exterms:approvedBy   ex:rulesCommittee .
ex:rulesCommittee  rdf:type             rdf:Bag .
ex:rulesCommittee  rdf:_1               ex:Fred .
ex:rulesCommittee  rdf:_2               ex:Wilma .
ex:rulesCommittee  rdf:_3               ex:Dino .

When using RDF containers, it is important to understand that the statements are not constructing containers, as in a programming language data structure. Instead, the statements are describing containers (groups of things) that presumably exist. For instance, in the Rules Committee example just given, the Rules Committee is an unordered group of people, whether it is described in RDF that way or not. Saying that the resource ex:rulesCommittee has type rdf:Bag is not saying that the Rules Committee is a data structure, or constructing a particular data structure to hold the members of the group (the Rules Committee could be described as a Bag without describing any members at all). Instead, it is describing the Rules Committee as having characteristics corresponding to those associated with a Bag container, namely that it has members, and their order of description is not significant. Similarly, using the container membership properties simply describes a container resource as having certain things as members. This does not necessarily say that the things described as members are the only members that exist. For example, the triples given above to describe the Rules Committee say only that Fred, Wilma, and Dino are members of the committee, not that they are the only members of the committee.

Also, Example 12 and Example 13 illustrated a common "pattern" in describing containers, regardless of the type of container involved (e.g., use of a blank node with an appropriate rdf:type property to represent the container itself, and use of rdf:_n to generate sequentially-numbered container membership properties). However, it is important to understand that RDF does not enforce this particular way of using the RDF container vocabulary, and so it is possible to use this vocabulary in other ways. For example, in some cases it might be appropriate to use a container resource having a URIref rather than using a blank node. Moreover, it is possible to use the container vocabulary in ways that may not describe graphs with the "well-formed" structures shown in the previous examples. For example, Example 14 shows the Turtle syntax for a graph similar to the Alt container shown in Figure 15:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix s:   <http://example.org/packages/vocab#>.

    s:DistributionSite [
        a rdf:Alt;
        a rdf:Bag;
        rdf:_2 <ftp://ftp.example.org>;
        rdf:_2 <ftp://ftp1.example.org>;
        rdf:_5 <ftp://ftp2.example.org>

As noted in [RDF-SEMANTICS], RDF imposes no "well-formedness" conditions on the use of the container vocabulary, so Example 14 is perfectly legal, even though the container is described as both a Bag and an Alt, it is described as having two distinct values of the rdf:_2 property, and it does not have rdf:_1, rdf:_3, or rdf:_4 properties.

As a result, RDF applications that require containers to be "well-formed" should be written to check that the container vocabulary is being used appropriately, in order to be fully robust.

4.2 RDF Collections

A limitation of the containers described in Section 4.1 is that there is no way to close them, i.e., to say "these are all the members of the container". As noted in Section 4.1, a container only says that certain identified resources are members; it does not say that other members do not exist. Also, while one graph may describe some of the members, there is no way to exclude the possibility that there is another graph somewhere that describes additional members. RDF provides support for describing groups containing only the specified members, in the form of RDF collections. An RDF collection is a group of things represented as a list structure in the RDF graph. This list structure is constructed using a predefined collection vocabulary consisting of the predefined type rdf:List, the predefined properties rdf:first and rdf:rest, and the predefined resource rdf:nil.

To illustrate this, the sentence "The students in course 6.001 are Amy, Mohamed, and Johann" could be represented using the graph shown in Figure 16:

In this graph, each member of the collection, such as s:Amy, is the object of an rdf:first property whose subject is a resource (a blank node in this example) that represents a list. This list resource is linked to the rest of the list by an rdf:rest property. The end of the list is indicated by the rdf:rest property having as its object the resource rdf:nil (the resource rdf:nil represents the empty list, and is defined as being of type rdf:List). This structure will be familiar to those who know the Lisp programming language. As in Lisp, the rdf:first and rdf:rest properties allow applications to traverse the structure. Each of the blank nodes forming this list structure is implicitly of type rdf:List (that is, each of these nodes implicitly has an rdf:type property whose value is the predefined type rdf:List), although this is not explicitly shown in the graph. The RDF Schema language [RDF-VOCABULARY] defines the properties rdf:first and rdf:rest as having subjects of type rdf:List, so the information about these nodes being lists can generally be inferred, rather than the corresponding rdf:type triples being written out all the time.

Turtle provides a special notation to make it easy to describe collections using graphs of this form. To illustrate how this notation works, the Turtle from Example 15 would result in the RDF graph shown in Figure 16:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix s:   <http://example.org/students/vocab#>.

    s:students (

The use of Turtle abbreviation for lists always defines a list structure like the one shown in Figure 16, i.e., a fixed finite list of items with a given length and terminated by rdf:nil, and which uses "new" blank nodes that are unique to the list structure itself. However, RDF does not enforce this particular way of using the RDF collection vocabulary, and so it is possible to use this vocabulary in other ways, some of which may not describe lists or closed collections. To see why, note that the graph shown in Figure 16 could also be written in Turtle by writing out the same triples "in longhand" (without using the special Turtle notation) using the collection vocabulary, as in Example 16:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix s:   <http://example.org/students/vocab#>.

<http://example.org/courses/6.001> s:student _:sch1.

    rdf:first <http://example.org/students/Amy>;
    rdf:rest  _:sch2.
    rdf:first <http://example.org/students/Mohamed>;
    rdf:rest  _:sch3.

    rdf:first <http://example.org/students/Johann>;
    rdf:rest  rdf:nil.

As noted in [RDF-SEMANTICS] (and as was the case for the container vocabulary described in Section 4.1), RDF imposes no "well-formedness" conditions on the use of the collection vocabulary so, when writing triples in longhand, it is possible to define RDF graphs with structures other than the well-structured graphs that would be automatically generated by using list notation of Turtle. For example, it is not illegal to assert that a given node has two distinct values of the rdf:first property, to create structures that have forked or non-list tails, or to simply omit part of the description of a collection. Also, graphs defined by using the collection vocabulary in longhand could use URIrefs to identify the components of the list instead of blank nodes unique to the list structure. In this case, it would be possible to create triples in other graphs that effectively added elements to the collection, making it non-closed.

As a result, RDF applications that require collections to be well-formed should be written to check that the collection vocabulary is being used appropriately, in order to be fully robust. In addition, languages such as OWL [OWL], which can define additional constraints on the structure of RDF graphs, can rule out some of these cases.

4.3 RDF Reification

RDF applications sometimes need to describe other RDF statements using RDF, for instance, to record information about when statements were made, who made them, or other similar information (this is sometimes referred to as "provenance" information). For example, Example 9 in Section 3.2 described a particular tent with URIref exproducts:item10245, offered for sale by example.com. One of the triples from that description, describing the weight of the tent, was:

exproducts:item10245   exterms:weight   "2.4"^^xsd:decimal .

and it might be useful for example.com to record who provided that particular piece of information.

RDF provides a built-in vocabulary intended for describing RDF statements. A description of a statement using this vocabulary is called a reification of the statement. The RDF reification vocabulary consists of the type rdf:Statement, and the properties rdf:subject, rdf:predicate, and rdf:object. However, while RDF provides this reification vocabulary, care is needed in using it, because it is easy to imagine that the vocabulary defines some things that are not actually defined. This point will be discussed further later in this section.

Using the reification vocabulary, a reification of the statement about the tent's weight would be given by assigning the statement a URIref such as exproducts:triple12345 (so statements can be written describing it), and then describing the statement using the statements:

exproducts:triple12345  rdf:type        rdf:Statement .
exproducts:triple12345  rdf:subject     exproducts:item10245 .
exproducts:triple12345  rdf:predicate   exterms:weight .
exproducts:triple12345  rdf:object      "2.4"^^xsd:decimal .

These statements say that the resource identified by the URIref exproducts:triple12345 is an RDF statement, that the subject of the statement refers to the resource identified by exproducts:item10245, the predicate of the statement refers to the resource identified by exterms:weight, and the object of the statement refers to the decimal value identified by the typed literal "2.4"^^xsd:decimal. Assuming that the original statement is actually identified by exproducts:triple12345, it should be clear by comparing the original statement with the reification that the reification actually does describe it. The conventional use of the RDF reification vocabulary always involves describing a statement using four statements in this pattern; the four statements are sometimes referred to as a "reification quad" for this reason.

Using reification according to this convention, example.com could record the fact that John Smith made the original statement about the tent's weight by first assigning the original statement a URIref (such as exproducts:triple12345 as before), describing that statement using the reification just described, and then adding an additional statement that exproducts:triple12345 was written by John Smith (using a URIref to identify which John Smith is being referred to). The resulting statements would be:

exproducts:triple12345   rdf:type        rdf:Statement .
exproducts:triple12345   rdf:subject     exproducts:item10245 .
exproducts:triple12345   rdf:predicate   exterms:weight .
exproducts:triple12345   rdf:object      "2.4"^^xsd:decimal .
exproducts:triple12345   dc:creator      exstaff:85740 . 

The original statement, together with the reification and the attribution of the statement to John Smith, forms the graph shown in Figure 17:

Section 3.2 introduced the use of the Turtle shorthand for URIrefs. This can also be used in a property element to automatically produce a reification of the triple that the property element generates. Example 17 shows how this could be used to produce the same graph :

In this case, specifying the attribute :triple12345 in the exterms:weight element results in the original triple describing the tent's weight:

exproducts:item10245   exterms:weight   "2.4"^^xsd:decimal .

plus the reification triples:

exproducts:triple12345   rdf:type        rdf:Statement .
exproducts:triple12345   rdf:subject     exproducts:item10245 .
exproducts:triple12345   rdf:predicate   exterms:weight .
exproducts:triple12345   rdf:object      "2.4"^^xsd:decimal .

The subject of these reification triples is a URIref formed by concatenating the base URI of the document (given in the @base declaration), the character "#" (to indicate that what follows is a fragment identifier), and the value of the shorthand; that is, the triples have the same subject exproducts:triple12345 as in the previous examples.

Note that asserting the reification is not the same as asserting the original statement, and neither implies the other. That is, when someone says that John said something about the weight of a tent, they are not making a statement about the weight of a tent themselves, they are making a statement about something John said. Conversely, when someone describes the weight of a tent, they are not also making a statement about a statement they made (since they may have no intention of talking about things called "statements").

The text above deliberately referred in a number of places to "the conventional use of reification". As noted earlier, care is needed when using the RDF reification vocabulary because it is easy to imagine that the vocabulary defines some things that are not actually defined. While there are applications that successfully use reification, they do so by following some conventions, and making some assumptions, that are in addition to the actual meaning that RDF defines for the reification vocabulary, and the actual facilities that RDF provides to support it.

For one thing, it is important to note that in the conventional use of reification, the subject of the reification triples is assumed to identify a particular instance of a triple in a particular RDF document, rather than some arbitrary triple having the same subject, predicate, and object. This particular convention is used because reification is intended for expressing properties such as dates of composition and source information, as in the examples given already, and these properties need to be applied to specific instances of triples. There could be several triples that have the same subject, predicate, and object and, although a graph is defined as a set of triples, several instances with the same triple structure might occur in different documents. Thus, to fully support this convention, there needs to be some means of associating the subject of the reification triples with an individual triple in some document. However, RDF provides no way to do this.

For instance, in the examples above, there is no explicit information in either the triples or the Turtle code that actually indicates that the original statement describing the tent's weight is the resource exproducts:triple12345, the resource that is the subject of the four reification statements and the statement that John Smith created it. This can be seen by looking at the drawn graph shown in Figure 17. The original statement is certainly part of this graph, but as far as the information in the graph is concerned, exproducts:triple12345 is a separate resource, rather than identifying that part of the graph. RDF does not provide a built-in way of indicating how a URIref like exproducts:triple12345 is associated with a particular statement or graph, any more than it provides a built-in way of indicating how a URIref like exproducts:item10245 is associated with an actual tent. Associating specific URIrefs with specific resources (statements in this case) must be done using mechanisms outside of RDF.

Using URIrefs shorthand as shown in Example 17 generates the reification automatically, and provides a convenient way of indicating the URIref to be used as the subject of the statements in the reification. Moreover, it provides a partial "hook" relating the triples in the reification with the piece of Turtle syntax that caused them to be created, since the value triple12345 of the shorthand is used to generate the URIref of the subject of the reification triples. However, this relationship is once again outside RDF, since there is nothing in the resulting triples that explicitly says that the original triple had the URIref exproducts:triple12345 (RDF does not assume there is any relationship between a URIref and any Turtle code that it might have been used or abbreviated in).

The lack of a built-in means for assigning URIrefs to statements does not mean that "provenance" information of this kind cannot be expressed in RDF, just that it cannot be done using only the meaning RDF associates with the reification vocabulary. For example, if an RDF document (say, a Web page) has a URI, statements could be made about the resource identified by that URI and, based on some application-dependent understanding of how those statements should be interpreted, an application could act as if those statements "distribute" over (apply equally to) all the statements in the document. Also, if some mechanism exists (outside of RDF) to assign URIs to individual RDF statements, then statements could certainly be made about those individual statements, using their URIs to identify them. However, in these cases, it would also not be strictly necessary to use the reification vocabulary in the conventional way.

To see this, assuming the original statement:

exproducts:item10245  exterms:weight  "2.4"^^xsd:decimal .

had a URIref of exproducts:triple12345, the statement could be attributed to John Smith simply by the statement:

exproducts:triple12345  dc:creator  exstaff:85740 .

with no use of the reification vocabulary (although the description of exproducts:triple12345 as having rdf:type rdf:Statement might also be helpful).

In addition, the reification vocabulary could be used directly according to the convention described above, along with an application-dependent understanding as to how to associate specific triples with their reifications. However, other applications receiving this RDF would not necessarily share this application-dependent understanding, and thus would not necessarily interpret the graphs appropriately.

It is also important to note that the interpretation of reification described here is not the same as "quotation", as found in some languages. Instead, the reification describes the relationship between a particular instance of a triple and the resources the triple refers to. The reification can be read intuitively as saying "this RDF triple talks about these things", rather than (as in quotation) "this RDF triple has this form." For instance, in the reification example used in this section, the triple:

exproducts:triple12345  rdf:subject  exproducts:item10245 .

describing the rdf:subject of the original statement says that the subject of the statement is the resource (the tent) identified by the URIref exproducts:item10245. It does not say that the subject of the statement is the URIref itself (i.e., a string beginning with certain characters), as quotation would do.

4.4 More on Structured Values: rdf:value

Section 2.3 noted that the RDF model intrinsically supports only binary relations; that is, a statement specifies a relation between two resources. For example, the statement:

exstaff:85740   exterms:manager   exstaff:62345 .

states that the relation exterms:manager holds between two employees (presumably one manages the other).

However, in some cases it is necessary to represent information involving higher arity relations (relations between more than two resources) in RDF. Section 2.3 discussed one example of this, where the problem was to represent the relationship between John Smith and his address information, and the value of John's address was a structured value of his street, city, state, and postal code. Writing this as a relation shows that this address is a 5-ary relation of the form:

address(exstaff:85740, "1501 Grant Avenue", "Bedford", "Massachusetts", "01730")

Section 2.3 noted that this kind of structured information can be represented in RDF by considering the aggregate thing be described (here, the group of components representing John's address) as a separate resource, and then making separate statements about that new resource, as in the triples:

exstaff:85740   exterms:address        _:johnaddress .
_:johnaddress   exterms:street         "1501 Grant Avenue" .
_:johnaddress   exterms:city           "Bedford" .
_:johnaddress   exterms:state          "Massachusetts" .
_:johnaddress   exterms:postalCode     "01730" .

(where _:johnaddress is the blank node identifier of the blank node representing John's address.)

This is a general way to represent any n-ary relation in RDF: select one of the participants (John in this case) to serve as the subject of the original relation (address in this case), then specify an intermediate resource to represent the rest of the relation (either with or without assigning it a URI), then give that new resource properties representing the remaining components of the relation.

In the case of John's address, none of the individual parts of the structured value could be considered the "main" value of the exterms:address property; all of the parts contribute equally to the value. However, in some cases one of the parts of the structured value is often thought of as the "main" value, with the other parts of the relation providing additional contextual or other information that qualifies the main value. For instance, in Example 9 in Section 3.2, the weight of a particular tent was given as the decimal value 2.4 using a typed literal, i.e.,

exproduct:item10245   exterms:weight   "2.4"^^xsd:decimal .

In fact, a more complete description of the weight would have been 2.4 kilograms rather than just the decimal value 2.4. To state this, the value of the exterms:weight property would need to have two components, the typed literal for the decimal value and an indication of the unit of measure (kilograms). In this situation the decimal value could be considered the "main" value of the exterms:weight property, because frequently the value would be recorded simply as the typed literal (as in the triple above), relying on an understanding of the context to fill in the unstated units information.

In the RDF model a qualified property value of this kind can be considered as simply another kind of structured value. To represent this, a separate resource could be used to represent the structured value as a whole (the weight, in this case), and to serve as the object of the original statement. That resource could then be given properties representing the individual parts of the structured value. In this case, there should be a property for the typed literal representing the decimal value, and a property for the unit. RDF provides a predefined rdf:value property to describe the main value (if there is one) of a structured value. So in this case, the typed literal could be given as the value of the rdf:value property, and the resource exunits:kilograms as the value of an exterms:units property (assuming the resource exunits:kilograms is defined as part of example.org's vocabulary). The resulting triples would be:

exproduct:item10245   exterms:weight   _:weight10245 .
_:weight10245         rdf:value        "2.4"^^xsd:decimal .
_:weight10245         exterms:units    exunits:kilograms .

The same approach can be used to represent quantities using any units of measure, as well as values taken from different classification schemes or rating systems, by using the rdf:value property to give the main value, and using additional properties to identify the classification scheme or other information that further describes the value.

There is no need to use rdf:value for these purposes (e.g., a user-defined property name, such as exterms:amount, could have been used instead of rdf:value), and RDF does not associate any special meaning with rdf:value. rdf:value is simply provided as a convenience for use in these commonly-occurring situations.

However, even though much existing data in databases and on the Web (and in later Primer examples) takes the form of simple values for properties such as weights, costs, etc., the principle that such simple values are often insufficient to adequately describe these values is an important one. In a global environment such as the Web, it is generally not safe to make the assumption that anyone accessing a property value will understand the units being used (or other contextually-dependent information that may be involved). For example, a U.S. site might give a weight value in pounds, but someone accessing that data from outside the U.S. might assume that weights are given in kilograms. The correct interpretation of data in the Web environment may require that additional information (such as units information) be explicitly recorded. This can be done in many ways, such as using rdf:value, building units into property names (e.g., exterms:weightInKg), defining specialized datatypes that include units information (e.g., extypes:kilograms), or adding additional user-defined properties to specify this information (e.g., exterms:unitOfWeight), either in descriptions of individual items or products, in descriptions of sets of data (e.g., all the data in a catalog or on a site), or in schemas (see Section 5).

5. Defining RDF Vocabularies: RDF Schema

RDF provides a way to express simple statements about resources, using named properties and values. However, RDF user communities also need the ability to define the vocabularies (terms) they intend to use in those statements, specifically, to indicate that they are describing specific kinds or classes of resources, and will use specific properties in describing those resources. For example, the company example.com from the examples in Section 3.2 would want to describe classes such as exterms:Tent, and use properties such as exterms:model, exterms:weightInKg, and exterms:packedSize to describe them (QNames with various "example" namespace prefixes are used as the names of classes and properties here as a reminder that in RDF these names are actually URI references, as discussed in Section 2.1). Similarly, people interested in describing bibliographic resources would want to describe classes such as ex2:Book or ex2:MagazineArticle, and use properties such as ex2:author, ex2:title, and ex2:subject to describe them. Other applications might need to describe classes such as ex3:Person and ex3:Company, and properties such as ex3:age, ex3:jobTitle, ex3:stockSymbol, and ex3:numberOfEmployees. RDF itself provides no means for defining such application-specific classes and properties. Instead, such classes and properties are described as an RDF vocabulary, using extensions to RDF provided by the RDF Vocabulary Description Language 1.0: RDF Schema [RDF-VOCABULARY], referred to here as RDF Schema.

RDF Schema does not provide a vocabulary of application-specific classes like exterms:Tent, ex2:Book, or ex3:Person, and properties like exterms:weightInKg, ex2:author or ex3:JobTitle. Instead, it provides the facilities needed to describe such classes and properties, and to indicate which classes and properties are expected to be used together (for example, to say that the property ex3:jobTitle will be used in describing a ex3:Person). In other words, RDF Schema provides a type system for RDF. The RDF Schema type system is similar in some respects to the type systems of object-oriented programming languages such as Java. For example, RDF Schema allows resources to be defined as instances of one or more classes. In addition, it allows classes to be organized in a hierarchical fashion; for example a class ex:Dog might be defined as a subclass of ex:Mammal which is a subclass of ex:Animal, meaning that any resource which is in class ex:Dog is also implicitly in class ex:Animal as well. However, RDF classes and properties are in some respects very different from programming language types. RDF class and property descriptions do not create a straightjacket into which information must be forced, but instead provide additional information about the RDF resources they describe. This information can be used in a variety of ways, which will be discussed in Section 5.3.

The RDF Schema facilities are themselves provided in the form of an RDF vocabulary; that is, as a specialized set of predefined RDF resources with their own special meanings. The resources in the RDF Schema vocabulary have URIrefs with the prefix http://www.w3.org/2000/01/rdf-schema# (conventionally associated with the QName prefix rdfs:). Vocabulary descriptions (schemas) written in the RDF Schema language are legal RDF graphs. Hence, RDF software that is not written to also process the additional RDF Schema vocabulary can still interpret a schema as a legal RDF graph consisting of various resources and properties, but will not "understand" the additional built-in meanings of the RDF Schema terms. To understand these additional meanings, RDF software must be written to process an extended language that includes not only the rdf: vocabulary, but also the rdfs: vocabulary, together with their built-in meanings. This point will be illustrated in the next section.

The following sections will illustrate RDF Schema's basic resources and properties.

5.1 Describing Classes

A basic step in any kind of description process is identifying the various kinds of things to be described. RDF Schema refers to these "kinds of things" as classes. A class in RDF Schema corresponds to the generic concept of a Type or Category, somewhat like the notion of a class in object-oriented programming languages such as Java. RDF classes can be used to represent almost any category of thing, such as Web pages, people, document types, databases or abstract concepts. Classes are described using the RDF Schema resources rdfs:Class and rdfs:Resource, and the properties rdf:type and rdfs:subClassOf.

For example, suppose an organization example.org wanted to use RDF to provide information about different kinds of motor vehicles. In RDF Schema, example.org would first need a class to represent the category of things that are motor vehicles. The resources that belong to a class are called its instances. In this case, example.org intends for the instances of this class to be resources that are motor vehicles.

In RDF Schema, a class is any resource having an rdf:type property whose value is the resource rdfs:Class. So the motor vehicle class would be described by assigning the class a URIref, say ex:MotorVehicle (using ex: to stand for the URIref http://www.example.org/schemas/vehicles, which is used as the prefix for URIrefs from example.org's vocabulary) and describing that resource with an rdf:type property whose value is the resource rdfs:Class. That is, example.org would write the RDF statement:

ex:MotorVehicle   rdf:type   rdfs:Class .

As indicated in Section 3.2, the property rdf:type is used to indicate that a resource is an instance of a class. So, having described ex:MotorVehicle as a class, resource exthings:companyCar would be described as a motor vehicle by the RDF statement:

exthings:companyCar   rdf:type   ex:MotorVehicle .

(This statement uses a common convention that class names are written with an initial uppercase letter, while property and instance names are written with an initial lowercase letter. However, this convention is not required in RDF Schema. The statement also assumes that example.org has decided to define separate vocabularies for classes of things, and instances of things.)

The resource rdfs:Class itself has an rdf:type of rdfs:Class. A resource may be an instance of more than one class.

After describing class ex:MotorVehicle, example.org might want to describe additional classes representing various specialized kinds of motor vehicle, e.g., passenger vehicles, vans, minivans, and so on. These classes can be described in the same way as class ex:MotorVehicle, by assigning a URIref for each new class, and writing RDF statements describing these resources as classes, e.g., writing:

ex:Van     rdf:type   rdfs:Class .
ex:Truck   rdf:type   rdfs:Class .

and so on. However, these statements by themselves only describe the individual classes. example.org may also want to indicate their special relationship to class ex:MotorVehicle, i.e., that they are specialized kinds of MotorVehicle.

This kind of specialization relationship between two classes is described using the predefined rdfs:subClassOf property to relate the two classes. For example, to state that ex:Van is a specialized kind of ex:MotorVehicle, example.org would write the RDF statement:

ex:Van   rdfs:subClassOf   ex:MotorVehicle .

The meaning of this rdfs:subClassOf relationship is that any instance of class ex:Van is also an instance of class ex:MotorVehicle. So if resource exthings:companyVan is an instance of ex:Van then, based on the declared rdfs:subClassOf relationship, RDF software written to understand the RDF Schema vocabulary can infer the additional information that exthings:companyVan is also an instance of ex:MotorVehicle.

This example of exthings:companyVan illustrates the point made earlier about RDF Schema defining an extended language. RDF itself does not define the special meaning of terms from the RDF Schema vocabulary such as rdfs:subClassOf. So if an RDF schema defines this rdfs:subClassOf relationship between ex:Van and ex:MotorVehicle, RDF software not written to understand the RDF Schema terms would recognize this as a triple, with predicate rdfs:subClassOf, but it would not understand the special significance of rdfs:subClassOf, and not be able to draw the additional inference that exthings:companyVan is also an instance of ex:MotorVehicle.

The rdfs:subClassOf property is transitive. This means, for example, that given the RDF statements:

ex:Van       rdfs:subClassOf   ex:MotorVehicle .
ex:MiniVan   rdfs:subClassOf   ex:Van .

RDF Schema defines ex:MiniVan as also being a subclass of ex:MotorVehicle. As a result, RDF Schema defines resources that are instances of class ex:MiniVan as also being instances of class ex:MotorVehicle (as well as being instances of class ex:Van). A class may be a subclass of more than one class (for example, ex:MiniVan may be a subclass of both ex:Van and ex:PassengerVehicle). RDF Schema defines all classes as subclasses of class rdfs:Resource (since the instances belonging to all classes are resources).

Figure 18 shows the full class hierarchy being discussed in these examples.

(To simplify the figure, the rdf:type properties relating each of the classes to rdfs:Class are omitted in Figure 18. In fact, RDF Schema defines both the subjects and objects of statements that use the rdfs:subClassOf property to be resources of type rdfs:Class, so this information could be inferred. However, in actually writing schemas, it is good practice to explicitly provide this information.)

This schema could also be described by the triples:

ex:MotorVehicle       rdf:type          rdfs:Class .
ex:PassengerVehicle   rdf:type          rdfs:Class .
ex:Van                rdf:type          rdfs:Class .
ex:Truck              rdf:type          rdfs:Class .
ex:MiniVan            rdf:type          rdfs:Class .

ex:PassengerVehicle   rdfs:subClassOf   ex:MotorVehicle .
ex:Van                rdfs:subClassOf   ex:MotorVehicle .
ex:Truck              rdfs:subClassOf   ex:MotorVehicle .

ex:MiniVan            rdfs:subClassOf   ex:Van .
ex:MiniVan            rdfs:subClassOf   ex:PassengerVehicle .

5.2 Describing Properties

In addition to describing the specific classes of things they want to describe, user communities also need to be able to describe specific properties that characterize those classes of things (such as rearSeatLegRoom to describe a passenger vehicle). In RDF Schema, properties are described using the RDF class rdf:Property, and the RDF Schema properties rdfs:domain, rdfs:range, and rdfs:subPropertyOf.

All properties in RDF are described as instances of class rdf:Property. So a new property, such as exterms:weightInKg, is described by assigning the property a URIref, and describing that resource with an rdf:type property whose value is the resource rdf:Property, for example, by writing the RDF statement:

exterms:weightInKg   rdf:type   rdf:Property .

RDF Schema also provides vocabulary for describing how properties and classes are intended to be used together in RDF data. The most important information of this kind is supplied by using the RDF Schema properties rdfs:range and rdfs:domain to further describe application-specific properties.

The rdfs:range property is used to indicate that the values of a particular property are instances of a designated class. For example, if example.org wanted to indicate that the property ex:author had values that are instances of class ex:Person, it would write the RDF statements:

ex:Person   rdf:type     rdfs:Class .
ex:author   rdf:type     rdf:Property .
ex:author   rdfs:range   ex:Person .

These statements indicate that ex:Person is a class, ex:author is a property, and that RDF statements using the ex:author property have instances of ex:Person as objects.

A property, say ex:hasMother, can have zero, one, or more than one range property. If ex:hasMother has no range property, then nothing is said about the values of the ex:hasMother property. If ex:hasMother has one range property, say one specifying ex:Person as the range, this says that the values of the ex:hasMother property are instances of class ex:Person. If ex:hasMother has more than one range property, say one specifying ex:Person as its range, and another specifying ex:Female as its range, this says that the values of the ex:hasMother property are resources that are instances of all of the classes specified as the ranges, i.e., that any value of ex:hasMother is both a ex:Female and a ex:Person.

This last point may not be obvious. However, stating that the property ex:hasMother has the two ranges ex:Female and ex:Person involves making two separate statements:

ex:hasMother   rdfs:range   ex:Female .
ex:hasMother   rdfs:range   ex:Person .

For any given statement using this property, say:

exstaff:frank   ex:hasMother   exstaff:frances .

in order for both the rdfs:range statements to be correct, it must be the case that exstaff:frances is both an instance of ex:Female and of ex:Person.

The rdfs:range property can also be used to indicate that the value of a property is given by a typed literal, as discussed in Section 2.4. For example, if example.org wanted to indicate that the property ex:age had values from the XML Schema datatype xsd:integer, it would write the RDF statements:

ex:age   rdf:type     rdf:Property .
ex:age   rdfs:range   xsd:integer .

The datatype xsd:integer is identified by its URIref (the full URIref being http://www.w3.org/2001/XMLSchema#integer). This URIref can be used without explicitly stating in the schema that it identifies a datatype. However, it is often useful to explicitly state that a given URIref identifies a datatype. This can be done using the RDF Schema class rdfs:Datatype. To state that xsd:integer is a datatype, example.org would write the RDF statement:

xsd:integer   rdf:type   rdfs:Datatype .

This statement says that xsd:integer is the URIref of a datatype (which is assumed to conform to the requirements for RDF datatypes described in [RDF-CONCEPTS]). Such a statement does not constitute a definition of a datatype, e.g., in the sense that example.org is defining a new datatype. There is no way to define datatypes in RDF Schema. As noted in Section 2.4, datatypes are defined externally to RDF (and to RDF Schema), and referred to in RDF statements by their URIrefs. This statement simply serves to document the existence of the datatype, and indicate explicitly that it is being used in this schema.

The rdfs:domain property is used to indicate that a particular property applies to a designated class. For example, if example.org wanted to indicate that the property ex:author applies to instances of class ex:Book, it would write the RDF statements:

ex:Book     rdf:type      rdfs:Class .
ex:author   rdf:type      rdf:Property .
ex:author   rdfs:domain   ex:Book .

These statements indicate that ex:Book is a class, ex:author is a property, and that RDF statements using the ex:author property have instances of ex:Book as subjects.

A given property, say exterms:weight, may have zero, one, or more than one domain property. If exterms:weight has no domain property, then nothing is said about the resources that exterms:weight properties may be used with (any resource could have a exterms:weight property). If exterms:weight has one domain property, say one specifying ex:Book as the domain, this says that the exterms:weight property applies to instances of class ex:Book. If exterms:weight has more than one domain property, say one specifying ex:Book as the domain and another one specifying ex:MotorVehicle as the domain, this says that any resource that has a exterms:weight property is an instance of all of the classes specified as the domains, i.e., that any resource that has a exterms:weight property is both a ex:Book and a ex:MotorVehicle (illustrating the need for care in specifying domains and ranges).

As in the case of rdfs:range, this last point may not be obvious. However, stating that the property exterms:weight has the two domains ex:Book and ex:MotorVehicle involves making two separate statements:

exterms:weight   rdfs:domain   ex:Book .
exterms:weight   rdfs:domain   ex:MotorVehicle .

For any given statement using this property, say:

exthings:companyCar   exterms:weight   "2500"^^xsd:integer .

in order for both the rdfs:domain statements to be correct, it must be the case that exthings:companyCar is both an instance of ex:Book and of ex:MotorVehicle.

The use of these range and domain descriptions can be illustrated by extending the vehicle schema, adding two properties ex:registeredTo and ex:rearSeatLegRoom, a new class ex:Person, and explicitly describing the datatype xsd:integer as a datatype. The ex:registeredTo property applies to any ex:MotorVehicle and its value is a ex:Person. For the sake of this example, ex:rearSeatLegRoom applies only to instances of class ex:PassengerVehicle. The value is an xsd:integer giving the number of centimeters of rear seat legroom. These descriptions are shown in Example 18 :

:registeredTo a rdf:Property;
    rdfs:domain :MotorVehicle;
    rdfs:range  :Person.

:rearSeatLegRoom a rdf:Property;
    rdfs:domain :PassengerVehicle;
    rdfs:range  xsd:integer.
:Person a rdfs:Class.
xsd:integer a rdfs:Datatype.                 

RDF Schema provides a way to specialize properties as well as classes. This specialization relationship between two properties is described using the predefined rdfs:subPropertyOf property. For example, if ex:primaryDriver and ex:driver are both properties, example.org could describe these properties, and the fact that ex:primaryDriver is a specialization of ex:driver, by writing the RDF statements:

ex:driver          rdf:type             rdf:Property .
ex:primaryDriver   rdf:type             rdf:Property .
ex:primaryDriver   rdfs:subPropertyOf   ex:driver .

The meaning of this rdfs:subPropertyOf relationship is that if an instance exstaff:fred is an ex:primaryDriver of the instance ex:companyVan, then RDF Schema defines exstaff:fred as also being an ex:driver of ex:companyVan.

A property may be a subproperty of zero, one or more properties. All RDF Schema rdfs:range and rdfs:domain properties that apply to an RDF property also apply to each of its subproperties. So, in the above example, RDF Schema defines ex:primaryDriver as also having an rdfs:domain of ex:MotorVehicle, because of its subproperty relationship to ex:driver.

Example 19 shows the Turtle for the full vehicle schema, containing all the descriptions given so far:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
@prefix xsd: <http://www.w3.org/2001/XMLSchema#>.
@base <http://example.org/schemas/vehicles>

:MotorVehicle a rdfs:Class.

:PassengerVehicle a rdfs:Class;
   rdfs:subClassOf :MotorVehicle.

:Truck a rdfs:Class;
   rdfs:subClassOf :MotorVehicle.
:Van a rdfs:Class;
   rdfs:subClassOf :MotorVehicle.

:MiniVan a rdfs:Class;
   rdfs:subClassOf :Van.
   rdfs:subClassOf :PassengerVehicle;

:Person a rdfs:Class.

xsd:integer a rdfs:Datatype.

:registeredTo a rdf:Property;
   rdfs:domain :MotorVehicle;
   rdfs:range  :Person.
:rearSeatLegRoom a rdf:Property;
   rdfs:domain rdf:resource :MotorVehicle;
   rdfs:range xsd:integer.

:driver a rdf:Property;
   rdfs:domain :MotorVehicle.

:primaryDriver a rdf:Property;
   rdfs:subPropertyOf :driver.

Having shown how to describe classes and properties using RDF Schema, instances using those classes and properties can now be illustrated. For example, Example 29 Example 20 describes an instance of the ex:PassengerVehicle class described in Example 19, together with some hypothetical values for its properties.

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
@prefix ex:  <http://example.org/schemas/vehicles#>
@base <http://example.org/things>

:johnSmithsCar a ex:PassengerVehicle;
    ex:registeredTo    <http://www.example.org/staffid/85740>;
    ex:rearSeatLegRoom "127"^^xsd:integer;
    ex:primaryDriver   <http://www.example.org/staffid/85740>.

This example assumes that the instance is described in a separate document from the schema. Since the schema has an xml:base of http://example.org/schemas/vehicles, the namespace declaration @prefix ex: <http://example.org/schemas/vehicles#> is provided to allow QNames such as ex:registeredTo in the instance data to be properly expanded to the URIrefs of the classes and properties described in that schema. An @base declaration is also provided for this instance, to allow :johnSmithsCar to expand to the proper URIref independently of the location of the actual document.

Note that an ex:registeredTo property can be used in describing this instance of ex:PassengerVehicle, because ex:PassengerVehicle is a subclass of ex:MotorVehicle. Note also that a typed literal is used for the value of the ex:rearSetLegRoom property in this instance, rather than a plain literal (i.e., rather than stating the value as ex:rearSeatLegRoom 127;). Because the schema describes the range of this property as an xsd:integer, the value of the property should be a typed literal of that datatype in order to match the range description (i.e., the range declaration does not automatically "assign" a datatype to a plain literal, and so a typed literal of the appropriate datatype must be explicitly provided). Additional information, either in the schema, or in additional instance data, could also be provided to explicitly specify the units of the ex:rearSetLegRoom property (centimeters), as discussed in Section 4.4.

5.3 Interpreting RDF Schema Declarations

As noted earlier, the RDF Schema type system is similar in some respects to the type systems of object-oriented programming languages such as Java. However, RDF differs from most programming language type systems in several important respects.

One important difference is that instead of describing a class as having a collection of specific properties, an RDF schema describes properties as applying to specific classes of resources, using domain and range properties. For example, a typical object-oriented programming language might define a class Book with an attribute called author having values of type Person. A corresponding RDF schema would describe a class ex:Book, and, in a separate description, a property ex:author having a domain of ex:Book and a range of ex:Person.

The difference between these approaches may seem to be only syntactic, but in fact there is an important difference. In the programming language class description, the attribute author is part of the description of class Book, and applies only to instances of class Book. Another class (say, softwareModule) might also have an attribute called author, but this would be considered a different attribute. In other words, the scope of an attribute description in most programming languages is restricted to the class or type in which it is defined. In RDF, on the other hand, property descriptions are, by default, independent of class definitions, and have, by default, global scope (although they may optionally be declared to apply only to certain classes using domain specifications).

As a result, an RDF schema could describe a property exterms:weight without a domain being specified. This property could then be used to describe instances of any class that might be considered to have a weight. One benefit of the RDF property-based approach is that it becomes easier to extend the use of property definitions to situations that might not have been anticipated in the original description. At the same time, this is a "benefit" which must be used with care, to insure that properties are not mis-applied in inappropriate situations.

Another result of the global scope of RDF property descriptions is that it is not possible in an RDF schema to define a specific property as having locally-different ranges depending on the class of the resource it is applied to. For example, in defining the property ex:hasParent, it would be desirable to be able to say that if the property is used to describe a resource of class ex:Human, then the range of the property is also a resource of class ex:Human, while if the property is used to describe a resource of class ex:Tiger, then the range of the property is also a resource of class ex:Tiger. This kind of definition is not possible in RDF Schema. Instead, any range defined for an RDF property applies to all uses of the property, and so ranges should be defined with care. However, while such locally-different ranges cannot be defined in RDF Schema, they can be defined in some of the richer schema languages discussed in Section 5.5.

Another important difference is that RDF Schema descriptions are not necessarily prescriptive in the way programming language type declarations typically are. For example, if a programming language declares a class Book with an author attribute having values of type Person, this is usually interpreted as a group of constraints. The language will not allow the creation of an instance of Book without an author attribute, and it will not allow an instance of Book with an author attribute that does not have a Person as its value. Moreover, if author is the only attribute defined for class Book, the language will not allow an instance of Book with some other attribute.

RDF Schema, on the other hand, provides schema information as additional descriptions of resources, but does not prescribe how these descriptions should be used by an application. For example, suppose an RDF schema states that an ex:author property has an rdfs:range of class ex:Person. This is simply an RDF statement that RDF statements containing ex:author properties have instances of ex:Person as objects.

This schema-supplied information might be used in different ways. One application might interpret this statement as specifying part of a template for RDF data it is creating, and use it to ensure that any ex:author property has a value of the indicated (ex:Person) class. That is, this application interprets the schema description as a constraint in the same way that a programming language might. However, another application might interpret this statement as providing additional information about data it is receiving, information which may not be provided explicitly in the original data. For example, this second application might receive some RDF data that includes an ex:author property whose value is a resource of unspecified class, and use this schema-provided statement to conclude that the resource must be an instance of class ex:Person. A third application might receive some RDF data that includes an ex:author property whose value is a resource of class ex:Corporation, and use this schema information as the basis of a warning that "there may be an inconsistency here, but on the other hand there may not be". Somewhere else there may be a declaration that resolves the apparent inconsistency (e.g., a declaration to the effect that "a Corporation is a (legal) Person").

Moreover, depending on how the application interprets the property descriptions, a description of an instance might be considered valid either without some of the schema-specified properties (e.g., there might be an instance of ex:Book without an ex:author property, even if ex:author is described as having a domain of ex:Book), or with additional properties (there might be an instance of ex:Book with an ex:technicalEditor property, even though the schema describing class ex:Book does not describe such a property).

In other words, statements in an RDF schema are always descriptions. They may also be prescriptive (introduce constraints), but only if the application interpreting those statements wants to treat them that way. All RDF Schema does is provide a way of stating this additional information. Whether this information conflicts with explicitly specified instance data is up to the application to determine and act upon.

5.4 Other Schema Information

RDF Schema provides a number of other built-in properties, which can be used to provide documentation and other information about an RDF schema or about instances. For example the rdfs:comment property can be used to provide a human-readable description of a resource. The rdfs:label property can be used to provide a more human-readable version of a resource's name. The rdfs:seeAlso property can be used to indicate a resource that might provide additional information about the subject resource. The rdfs:isDefinedBy property is a subproperty of rdfs:seeAlso, and can be used to indicate a resource that (in a sense not specified by RDF; e.g., the resource may not be an RDF schema) "defines" the subject resource. RDF Vocabulary Description Language 1.0: RDF Schema [RDF-VOCABULARY] should be consulted for further discussion of these properties.

As with a number of the built-in RDF properties such as rdf:value, the uses described for these RDF Schema properties are only their intended uses. [RDF-SEMANTICS] defines no special meanings for these properties, and RDF Schema does not define any constraints based on these intended uses. For example, there is no constraint specified that the object of a rdfs:seeAlso property must provide additional information about the subject of the statement in which it appears.

5.5 Richer Schema Languages

RDF Schema provides basic capabilities for describing RDF vocabularies, but additional capabilities are also possible, and can be useful. These capabilities may be provided through further development of RDF Schema, or in other languages based on RDF. Other richer schema capabilities that have been identified as useful (but that are not provided by RDF Schema) include:

The additional capabilities mentioned above, in addition to others, are the targets of ontology languages such as DAML+OIL [DAML+OIL] and OWL [OWL]. Both these languages are based on RDF and RDF Schema (and both currently provide all the additional capabilities mentioned above). The intent of such languages is to provide additional machine-processable semantics for resources, that is, to make the machine representations of resources more closely resemble their intended real world counterparts. While such capabilities are not necessarily needed to build useful applications using RDF (see Section 6 for a description of a number of existing RDF applications), the development of such languages is a very active subject of work as part of the development of the Semantic Web.

6. Some RDF Applications: RDF in the Field

The previous sections have described the general capabilities of RDF and RDF Schema. While examples were used in those sections to illustrate those capabilities, and some of those examples may have suggested potential RDF applications, those sections did not actually discuss any real applications. This section will describe some actual deployed RDF applications, showing how RDF supports various real-world requirements to represent and manipulate information about a wide variety of things.

6.1 Dublin Core Metadata Initiative

Metadata is data about data. Specifically, the term refers to data used to identify, describe, or locate information resources, whether these resources are physical or electronic. While structured metadata processed by computers is relatively new, the basic concept of metadata has been used for many years in helping manage and use large collections of information. Library card catalogs are a familiar example of such metadata.

The Dublin Core is a set of "elements" (properties) for describing documents (and hence, for recording metadata). The element set was originally developed at the March 1995 Metadata Workshop in Dublin, Ohio. The Dublin Core has subsequently been modified on the basis of later Dublin Core Metadata workshops, and is currently maintained by the Dublin Core Metadata Initiative. The goal of the Dublin Core is to provide a minimal set of descriptive elements that facilitate the description and the automated indexing of document-like networked objects, in a manner similar to a library card catalog. The Dublin Core metadata set is intended to be suitable for use by resource discovery tools on the Internet, such as the "Webcrawlers" employed by popular World Wide Web search engines. In addition, the Dublin Core is meant to be sufficiently simple to be understood and used by the wide range of authors and casual publishers who contribute information to the Internet. Dublin Core elements have become widely used in documenting Internet resources (the Dublin Core creator element has already been used in earlier examples). The current elements of the Dublin Core are defined in the Dublin Core Metadata Element Set, Version 1.1: Reference Description [DC], and contain definitions for the following properties:

Information using the Dublin Core elements may be represented in any suitable language (e.g., in HTML meta elements). However, RDF is an ideal representation for Dublin Core information. The examples below represent the simple description of a set of resources in RDF using the Dublin Core vocabulary. Note that the specific Dublin Core RDF vocabulary shown here is not intended to be authoritative. The Dublin Core Reference Description [DC] is the authoritative reference.

The first example, Example 21, describes a Web site home page using Dublin Core properties:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.               
@prefix dc:  <http://purl.org/dc/elements/1.1/>.        

    dc:title       "D-Lib Program - Research in Digital Libraries";
    dc:description """The D-Lib program supports the community of people
       with research interests in digital libraries and electronic
    dc:publisher:  "Corporation For National Research Initiatives";
    dc:date        "1995-01-07";
    dc:subject [
         a rdf:Bag;
         rdf:_1 "Research; statistical methods";
         rdf:_2 "Education, research, related topics";
         rdf:_3 "Library use Studies".
    dc:type        "World Wide Web Home Page";
    dc:format      "text/html";
    dc:language    "en".

Note that both RDF and the Dublin Core define an (XML) element called "Description" (although the Dublin Core element name is written in lowercase). Even if the initial letter were identically uppercase, the XML namespace mechanism enables these two elements to be distinguished (one is rdf:Description, and the other is dc:description). Also, as a matter of interest, accessing http://purl.org/dc/elements/1.1/ (the namespace URI used to identify the Dublin Core vocabulary in this example) in a Web browser (as of the current writing) will retrieve an RDF Schema declaration for [DC].

The second example, Example 22, describes a published magazine:

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.               
@prefix dc: <http://purl.org/dc/elements/1.1/>.        
@prefix dcterms: <http://purl.org/dc/terms/>.        

      dc:title         "DLIB Magazine - The Magazine for Digital Library Research - May 1998";
      dc:description   """D-LIB magazine is a monthly compilation of
       contributed stories, commentary, and briefings.""";
      dc:contributor   "Amy Friedlander";
      dc:publisher     "Corporation for National Research Initiatives";
      dc:date          "1998-01-05";
      dc:type          "electronic journal";
      dc:subject [
        a rdf:Bag;
        rdf:_1 "library use studies";
        rdf:_2 "magazines and newspapers".
      dc:format        "text/html";
      dc:identifier    <urn:issn:1082-9873>;
      dcterms:isPartOf <http://www.dlib.org>.

Example 22 uses (in the last line) the Dublin Core qualifier isPartOf (from a separate vocabulary) to indicate that this magazine is "part of" the previously-described Web site.

The third example, Example 23, describes a specific article in the magazine described in Example 22.

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.               
@prefix dc: <http://purl.org/dc/elements/1.1/>.        
@prefix dcterms: <http://purl.org/dc/terms/>.        

   dc:title         "An Introduction to the Resource Description Framework";
   dc:creator       "Eric J. Miller";
      """The Resource Description Framework (RDF) is an
       infrastructure that enables the encoding, exchange and reuse of
       structured metadata. rdf is an application of xml that imposes needed
       structural constraints to provide unambiguous methods of expressing
       semantics. rdf additionally provides a means for publishing both
       human-readable and machine-processable vocabularies designed to
       encourage the reuse and extension of metadata semantics among
       disparate information communities. the structural constraints rdf
       imposes to support the consistent encoding and exchange of
       standardized metadata provides for the interchangeability of separate
       packages of metadata defined by different resource description
    dc:publisher     "Corporation for National Research Initiatives";
    dc:subject [
        a rdf:Bag;
        rdf:_1 "machine-readable catalog record formats";
        rdf:_2 "applications of computer file organization and access methods".
    dc:rights        "Copyright © 1998 Eric Miller";
    dc:type          "Electronic Document";
    dc:format        "text/html";
    dc:language      "en";
    dcterms:isPartOf <http://www.dlib.org/dlib/may98/05contents.html>.

Example 23 also uses the qualifier isPartOf, this time to indicate that this article is "part of" the previously-described magazine.


PRISM: Publishing Requirements for Industry Standard Metadata [PRISM] is a metadata specification developed in the publishing industry. Magazine publishers and their vendors formed the PRISM Working Group to identify the industry's needs for metadata and define a specification to meet them. Publishers want to use existing content in many ways in order to get a greater return on the investment made in creating it. Converting magazine articles to HTML for posting on the Web is one example. Licensing it to aggregators like LexisNexis is another. All of these are "first uses" of the content; typically they all go live at the time the magazine hits the stands. The publishers also want their content to be "evergreen". It might be used in new issues, such as in a retrospective article. It could be used by other divisions in the company, such as in a book compiled from the magazine's photos, recipes, etc. Another use is to license it to outsiders, such as in a reprint of a product review, or in a retrospective produced by a different publisher. This overall goal requires a metadata approach that emphasizes discovery, rights tracking, and end-to-end metadata.

Discovery: Discovery is a general term for finding content which encompasses searching, browsing, content routing, and other techniques. Discussions of discovery frequently center on a consumer searching a public Web site. However, discovering content is much broader than that. The audience may consist of consumers, or it may consist of internal users such as researchers, designers, photo editors, licensing agents, etc. To assist discovery, PRISM provides properties to describe the topics, formats, genre, origin, and contexts of a resource. It also provides means for categorizing resources using multiple subject description taxonomies.

Rights Tracking: Magazines frequently contain material licensed from others. Photos from a stock photo agency are the most common type of licensed material, but articles, sidebars, and all other types of content may be licensed. Simply knowing if content was licensed for one-time use, requires royalty payments, or is wholly-owned by the publisher is a struggle. PRISM provides elements for basic tracking of such rights. A separate vocabulary defined in the PRISM specification supports description of places, times, and industries where content may or may not be used.

End-to-end metadata: Most published content already has metadata created for it. Unfortunately, when content moves between systems, the metadata is frequently discarded, only to be re-created later in the production process at considerable expense. PRISM aims to reduce this problem by providing a specification that can be used in multiple stages in the content production pipeline. An important feature of the PRISM specification is its use of other existing specifications. Rather than create an entirely new thing, the group decided to use existing specifications as much as possible, and only define new things where needed. For this reason, the PRISM specification uses XML, RDF, Dublin Core, and well as various ISO formats and vocabularies.

A PRISM description may be as simple as a few Dublin Core properties with plain literal values. Example 24 describes a photograph, giving basic information on its title, photographer, format, etc.

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix dc: <http://purl.org/dc/elements/1.1/>.

    dc:title       "Walking on the Beach in Corfu";
    dc:description "Photograph taken at 6:00 am on Corfu with two models";
    dc:creator     "John Peterson";
    dc:contributor "Sally Smith, lighting";
    dc:format      "image/jpeg".

PRISM also augments the Dublin Core to allow more detailed descriptions. The augmentations are defined as three new vocabularies, generally cited using the prefixes prism:, pcv:, and prl:.

prism: This prefix refers to the main PRISM vocabulary, whose terms use the URI prefix http://prismstandard.org/namespaces/basic/1.0/ . Most of the properties in this vocabulary are more specific versions of properties from the Dublin Core. For example, more specific versions of dc:date are provided by properties like prism:publicationTime, prism:releaseTime, prism:expirationTime, etc.

pcv: This prefix refers to the PRISM Controlled Vocabulary (pcv) vocabulary, whose terms use the URI prefix http://prismstandard.org/namespaces/pcv/1.0/ . Currently, common practice for describing the subject(s) of an article is by supplying descriptive keywords. Unfortunately, simple keywords do not make a great difference in retrieval performance, due to the fact that different people will use different keywords [BATES96]. Best practice is to code the articles with subject terms from a "controlled vocabulary". The vocabulary should provide as many synonyms as possible for its terms in the vocabulary. This way the controlled terms provide a meeting ground for the keywords supplied by the searcher and the indexer. The pcv vocabulary provides properties for specifying terms in a vocabulary, the relations between terms, and alternate names for the terms.

prl: This prefix refers to the PRISM Rights Language vocabulary, whose terms use the URI prefix http://prismstandard.org/namespaces/prl/1.0/ . Digital Rights Management is an area undergoing considerable upheaval. There are a number of proposals for rights management languages, but none are clearly favored throughout the industry. Because there was no clear choice to recommend, the PRISM Rights Language (PRL) was defined as an interim measure. It provides properties which let people say if an item can or cannot be "used", depending on conditions of time, geography, and industry. This is believed to be an 80/20 trade-off which will help publishers begin to save money when tracking rights. It is not intended to be a general rights language, or allow publishers to automatically enforce limits on consumer uses of the content.

PRISM uses RDF because of its abilities for dealing with descriptions of varying complexity. Currently, a great deal of metadata uses simple character string (plain literal) values, such as:

dc:coverage "Greece";

Over time the developers of PRISM expect uses of the PRISM specification to become more sophisticated, moving from simple literal values to more structured values. In fact, that range of values is a situation being faced now. Some publishers already use sophisticated controlled vocabularies, others are barely using manually-supplied keywords. To illustrate this, some examples of the different kinds of values that can be given for the dc:coverage property are:

dc:coverage "Greece";
dc:coverage <http://prismstandard.org/vocabs/ISO-3166/GR>;

(i.e., using either a plain literal or a URIref to identify the country) and

dc:coverage <http://prismstandard.org/vocabs/ISO-3166/GR>.
<http://prismstandard.org/vocabs/ISO-3166/GR> rdf:type pcv:Descriptor;
<http://prismstandard.org/vocabs/ISO-3166/GR> pcv:label "Greece"@en;
<http://prismstandard.org/vocabs/ISO-3166/GR> pcv:label "Grèce"@fr.

(using a structured value to provide both a URIref and names in various languages).

Note also that there are properties whose meanings are similar, or subsets of other properties. For example, the geographic subject of a resource could be given with

prism:subject "Greece";
dc:coverage   "Greece";


prism:location "Greece";

Any of those properties might use the simple literal value, or a more complex structured value. Such a range of possibilities cannot be adequately described by DTDs, or even by the newer XML Schemas. While there is a wide range of syntactic variations to deal with, RDF's graph model has a simple structure - a set of triples. Dealing with the metadata in the triples domain makes it much easier for older software to accommodate content with new extensions.

This section closes with two final examples. Example 25 says that the image (.../Corfu.jpg) cannot be used (#none) in the tobacco industry (code 21 in SIC, the Standard Industrial Classifications).

@prefix prism: <http://prismstandard.org/namespaces/basic/1.0/>.
@prefix prl:   <http://prismstandard.org/namespaces/prl/1.0/>.
@prefix rdf:   <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix dc:    <http://purl.org/dc/elements/1.1/>.

    dc:rights [
      prl:usage    <http://prismstandard.org/vocabularies/1.0/usage.xml#none>;
      prl:industry <http://prismstandard.org/vocabs/SIC/21>.

Example 26 says that the photographer for the Corfu image was employee 3845, better known as John Peterson. It also says that the geographic coverage of the photo is Greece. It does so by providing, not just a code from a controlled vocabulary, but a cached version of the information for that term in the vocabulary.

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix pcv: <http://prismstandard.org/namespaces/pcv/1.0/>.
@prefix dc:  <http://purl.org/dc/elements/1.1/>.

   dc:identifier <http://travel.example.com/content/2357845> ;
   dc:creator    <http://travel.example.com/content/2357845> ;
   dc:coverage   <http://prismstandard.org/vocabs/ISO-3166/GR>.
</content/2357845> a pcv:Descriptor;
   pcv:label "John Peterson".
   pcv:label "Greece"@en;
   pcv:label "Grèce"@fr.

7. Other Parts of the RDF Specification

Section 1 indicated that the RDF Specification consists of a number of documents:

This Primer has already discussed the subjects of several of these documents, basic RDF concepts (in Section 2) and RDF Schema (in Section 5). This section briefly describes the remaining documents (even though there have already been numerous references to [RDF-SEMANTICS] as well), in order to explain their role in the complete specification of RDF.

7.1 RDF Semantics

As discussed in the preceding sections, RDF is intended to be used to express statements about resources in the form of a graph, using specific vocabularies (names of resources, properties, classes, etc.). RDF is also intended to be the foundation for more advanced languages, such as those discussed in Section 5.5. In order to serve these purposes, the "meaning" of an RDF graph must be defined in a very precise manner.

Exactly what constitutes the "meaning" of an RDF graph in a very general sense may depend on many factors, including conventions within a user community to interpret user-defined RDF classes and properties in specific ways, comments in natural language, or links to other content-bearing documents. As noted briefly in Section 2.2, much of the meaning conveyed in these forms will not be directly accessible to machine processing, although this meaning may be used by human interpreters of the RDF information, or by programmers writing software to perform various kinds of processing on that RDF information. However, RDF statements also have a formal meaning which determines, with mathematical precision, the conclusions (or entailments) that machines can draw from a given RDF graph. The RDF Semantics [RDF-SEMANTICS] document defines this formal meaning, using a technique called model theory for specifying the semantics of a formal language. [RDF-SEMANTICS] also defines the semantic extensions to the RDF language represented by RDF Schema, and by individual datatypes. In other words, the RDF model theory provides the formal underpinnings for all RDF concepts. Based on the semantics defined in the model theory, it is simple to translate an RDF graph into a logical expression with essentially the same meaning.

7.2 Test Cases

The RDF Test Cases [RDF-TESTS] supplement the textual RDF specifications with test cases (examples) corresponding to particular technical issues addressed by the RDF Core Working Group. To help describe these examples, the Test Cases document introduces a notation called N-Triples, which provides the basis for the triples notation used throughout this Primer. The test cases are published in machine-readable form at Web locations referenced by the Test Cases document, so developers can use these as the basis for automated testing of RDF software.

The test cases are divided into a number of categories:

The test cases are not a complete specification of RDF, and are not intended to take precedence over the other specification documents. However, they are intended to illustrate the intent of the RDF Core Working Group with respect to the design of RDF, and developers may find these test cases helpful should the wording of the specifications be unclear on any point of detail.

8. References

8.1 Normative References

Resource Description Framework (RDF): Concepts and Abstract Syntax , Klyne G., Carroll J. (Editors), W3C Recommendation, 10 February 2004. This version is http://www.w3.org/TR/2004/REC-rdf-primer-20040210/. The latest version is http://www.w3.org/TR/rdf-concepts/.
MIME Media Types , The Internet Assigned Numbers Authority (IANA). This document is http://www.iana.org/assignments/media-types/ . The registration for application/rdf+xml is archived at http://www.w3.org/2001/sw/RDFCore/mediatype-registration .
Resource Description Framework (RDF) Model and Syntax Specification , Lassila O., Swick R. (Editors), World Wide Web Consortium, 22 February 1999. This version is http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/. The latest version is http://www.w3.org/TR/REC-rdf-syntax/.
RDF Primer , Manola F., Miller E. (Editors), W3C Recommendation, 10 February 2004. This version is http://www.w3.org/TR/2004/REC-rdf-primer-20040210/. The latest version is http://www.w3.org/TR/rdf-primer/.
RDF Semantics , Hayes P. (Editor), W3C Recommendation, 10 February 2004. This version is http://www.w3.org/TR/2004/REC-rdf-mt-20040210/. The latest version is http://www.w3.org/TR/rdf-mt/.
RDF/XML Syntax Specification (Revised) , Beckett D. (Editor), W3C Recommendation, 10 February 2004. This version http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/. The latest version is http://www.w3.org/TR/rdf-syntax-grammar/.
RDF Test Cases , Grant J., Beckett D. (Editors), W3C Recommendation, 10 February 2004. This version is http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/. The latest version is http://www.w3.org/TR/rdf-testcases/.
RDF Vocabulary Description Language 1.0: RDF Schema , Brickley D., Guha R.V. (Editors), W3C Recommendation, 10 February 2004. This version is http://www.w3.org/TR/2004/REC-rdf-schema-20040210/. The latest version is http://www.w3.org/TR/rdf-schema/.
The Unicode Standard, Version 3, The Unicode Consortium, Addison-Wesley, 2000. ISBN 0-201-61633-5, as updated from time to time by the publication of new versions. (See http://www.unicode.org/unicode/standard/versions/ for the latest version and additional information on versions of the standard and of the Unicode Character Database).
RFC 2396 - Uniform Resource Identifiers (URI): Generic Syntax , Berners-Lee T., Fielding R., Masinter L., IETF, August 1998, http://www.isi.edu/in-notes/rfc2396.txt.
Extensible Markup Language (XML) 1.0, Second Edition , Bray T., Paoli J., Sperberg-McQueen C.M., Maler E. (Editors), World Wide Web Consortium, 6 October 2000. This version is http://www.w3.org/TR/2000/REC-xml-20001006. The latest version is http://www.w3.org/TR/REC-xml.

8.2 Informational References

Addressing Schemes , Connolly D., 2001. This document is http://www.w3.org/Addressing/schemes.html.
Indexing and Access for Digital Libraries and the Internet: Human, Database, and Domain Factors , Bates M.J., 1996. This document is http://is.gseis.ucla.edu/research/mjbates.html.
What the Semantic Web can represent , Berners-Lee T., 1998. This document is http://www.w3.org/DesignIssues/RDFnot.html.
Conceptual Graphs, Sowa J., ISO working document ISO/JTC1/SC32/WG2 N 000, 2 April 2001 (work in progress). Available at http://users.bestweb.net/~sowa/cg/cgstand.htm.
DAML+OIL (March 2001) Reference Description , Connolly D., van Harmelen F., Horrocks I., McGuinness D.L., Patel-Schneider P.F., Stein L.A., World Wide Web Consortium, 18 December 2001. This document is http://www.w3.org/TR/daml+oil-reference.
Dublin Core Metadata Element Set, Version 1.1: Reference Description , 02 June 2003. This version is http://dublincore.org/documents/2003/06/02/dces/. The latest version is http://dublincore.org/documents/dces/.
Logic, Algebra and Databases, Gray P., Ellis Horwood Ltd., 1984. ISBN 0-85312-709-3, 0-85312-803-0, 0-470-20103-7, 0-470-20259-9.
In Defense of Logic, Hayes P., Proceedings from the International Joint Conference on Artificial Intelligence, 1975, San Francisco. Morgan Kaufmann Inc., 1977. Also in Computation and Intelligence: Collected Readings, Luger G. (ed), AAAI press/MIT press, 1995. ISBN 0-262-62101-0.
Knowledge Interchange Format, Genesereth M., draft proposed American National Standard NCITS.T2/98-004. Available at http://logic.stanford.edu/kif/dpans.html.
Artificial Intelligence: Structures and Strategies for Complex Problem Solving (3rd ed.), Luger G., Stubblefield W., Addison Wesley Longman, 1998. ISBN 0-805-31196-3.
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PRISM: Publishing Requirements for Industry Standard Metadata , Version 1.1, 19 February 2002. The latest version of the PRISM specification is available at http://www.prismstandard.org/.
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@@@To be filled in@@@
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9. Acknowledgments

The original, RDF/XML version of this document has benefited from inputs from many members of the RDF Core Working Group. Specific thanks are due to Art Barstow, Dave Beckett, Dan Brickley, Ron Daniel, Ben Hammersley, Martyn Horner, Graham Klyne, Sean Palmer, Patrick Stickler, Aaron Swartz, Ralph Swick, and Garret Wilson who, together with the many people who commented on earlier versions of the Primer, provided valuable contributions to this document.

In addition, this document contains a significant contribution from Pat Hayes, Sergey Melnik, and Patrick Stickler, who led the development of the RDF datatype facilities described in the RDF family of specifications.

Frank Manola also thanks The MITRE Corporation, Frank's employer during most of the preparation of this document, for its support of his RDF Core Working Group activities under a MITRE Sponsored Research grant.

Appendix A: More on Uniform Resource Identifiers (URIs)

Note: This section is intended to provide a brief introduction to URIs. The definitive specification of URIs is RFC 2396 [URIS], which should be consulted for further details. Additional discussion of URIs can also be found in Naming and Addressing: URIs, URLs, ... [NAMEADDRESS].

As discussed in Section 2.1, the Web provides a general form of identifier, called the Uniform Resource Identifier (URI), for identifying (naming) resources on the Web. Unlike URLs, URIs are not limited to identifying things that have network locations, or use other computer access mechanisms. A number of different URI schemes (URI forms) have been already been developed, and are being used, for various purposes. Examples include:

A list of existing URI schemes can be found in Addressing Schemes [ADDRESS-SCHEMES], and it is a good idea to consider adapting one of the existing schemes for any specialized identification purposes, rather than trying to invent a new one.

No one person or organization controls who makes URIs or how they can be used. While some URI schemes, such as URL's http:, depend on centralized systems such as DNS, other schemes, such as freenet:, are completely decentralized. This means that, as with any other kind of name, no one needs special authority or permission to create a URI for something. Also, anyone can create URIs to refer to things they do not own, just as in ordinary language anyone can use whatever name they like for things they do not own.

As also noted in Section 2.1, RDF uses URI references [URIS] to name subjects, predicates, and objects in RDF statements. A URI reference (or URIref) is a URI, together with an optional fragment identifier at the end. For example, the URI reference http://www.example.org/index.html#section2 consists of the URI http://www.example.org/index.html and (separated by the "#" character) the fragment identifier Section2. RDF URIrefs can contain Unicode [UNICODE] characters (see [RDF-CONCEPTS]), allowing many languages to be reflected in URIrefs.

URIrefs may be either absolute or relative. An absolute URIref refers to a resource independently of the context in which the URIref appears, e.g., the URIref http://www.example.org/index.html. A relative URIref is a shorthand form of an absolute URIref, where some prefix of the URIref is missing, and information from the context in which the URIref appears is required to fill in the missing information. For example, the relative URIref otherpage.html, when appearing in a resource http://www.example.org/index.html, would be filled out to the absolute URIref http://www.example.org/otherpage.html. A URIref without a URI part is considered a reference to the current document (the document in which it appears). So, an empty URIref within a document is considered equivalent to the URIref of the document itself. A URIref consisting of just a fragment identifier is considered equivalent to the URIref of the document in which it appears, with the fragment identifier appended to it. For example, within http://www.example.org/index.html, if #section2 appeared as a URIref, it would be considered equivalent to the absolute URIref http://www.example.org/index.html#section2.

[RDF-CONCEPTS] notes that RDF graphs (the abstract models) do not use relative URIrefs, i.e., the subjects, predicates, and objects (and datatypes in typed literals) in RDF statements must always be identified independently of any context. However, a specific concrete RDF syntax, such as RDF/XML or Turtle, may allow relative URIrefs to be used as a shorthand for absolute URIrefs in certain situations. Turtle does permit such use of relative URIrefs, and some of the Turtle examples in this Primer illustrate such uses. [TURTLE] should be consulted for further details.

Both RDF and Web browsers use URIrefs to identify things. However, RDF and browsers interpret URIrefs in slightly different ways. This is because RDF uses URIrefs only to identify things, while browsers also use URIrefs to retrieve things. Often there is no effective difference, but in some cases the difference can be significant. One obvious difference is that when a URIref is used in a browser, there is the expectation that it identifies a resource that can actually be retrieved: that something is actually "at" the location identified by the URI. However, in RDF a URIref may be used to identify something, such as a person, that cannot be retrieved on the Web. People sometimes use RDF together with a convention that, when a URIref is used to identify an RDF resource, a page containing descriptive information about that resource will be placed on the Web "at" that URI, so that the URIref can be used in a browser to retrieve that information. This can be a useful convention in some circumstances, although it creates a difficulty in distinguishing the identity of the original resource from the identity of the Web page describing it (a subject discussed further in Section 2.3). However, this convention is not an explicit part of the definition of RDF, and RDF itself does not assume that a URIref identifies something that can be retrieved.

Another difference is in the way URIrefs with fragment identifiers are handled. Fragment identifiers are often seen in the URLs that identify HTML documents, where they serve to identify a specific place within the document identified by the URL. In normal HTML usage, where URI references are used to retrieve the indicated resources, the two URIrefs:


are related (they both refer to the same document, the second one identifying a location within the first one). However, as noted already, RDF uses URI references purely to identify resources, not to retrieve them, and RDF assumes no particular relationship between these two URIrefs. As far as RDF is concerned, they are syntactically different URI references, and hence may refer to unrelated things. This does not mean that the HTML-defined containment relationship might not exist, just that RDF does not assume that a relationship exists based only on the fact that the URI parts of the URI references are the same.

Carrying this point further, RDF does not assume that there is any relationship between URI references that share a common leading string, whether there is a fragment identifier or not. For example, as far as RDF is concerned, the two URIrefs:


have no particular relationship even though both of them start with the string http://www.example.org/. To RDF, they are simply different resources, because their URIrefs are different. (They may in fact be two files located in the same directory, but RDF does not assume this or any other relationship exists.)

RDF/XML Metadata