N.B. This document series is now deprecated. Please see http://www.w3.org/2004/02/skos/core/guide/ for the latest version of the SKOS Core Guide.

Abstract:
This document is a guide to using the SKOS-Core 1.0 RDF schema for thesauri. The SKOS meta-model is introduced, the features of the SKOS-Core 1.0 schema are described with examples, and guidelines for using SKOS concept schemes for subject-based indexing of web resources are offered.
Project name:
Semantic Web Advanced Development for Europe (SWAD-Europe)
Project Number:
IST-2001-34732
Workpackage name:
8. Thesaurus Research Prototype
Workpackage description:
http://www.w3.org/2001/sw/Europe/plan/workpackages/live/esw-wp-8.html 
Deliverable title:
8.1: thesaurus_schema_draft
This version:
http://www.w3.org/2001/sw/Europe/reports/thes/8.1/version05.html
Latest version:
http://www.w3.org/2001/sw/Europe/reports/thes/8.1/
Previous version:
http://www.w3.org/2001/sw/Europe/reports/thes/8.1/version04.html
Status:
Completed
Authors:
Alistair J. Miles, CCLRC
Nikki Rogers, ILRT
Dave Beckett, ILRT

Status of this document

This section describes the status of this document at the time of its publication. This is a draft document and may be updated, replaced, or obsoleted by other documents at any time. The latest status of this document series is maintained at the W3C.

This document is a public DRAFT for discussion. This guide and the SKOS-Core schema are an output of the research work of the Semantic Web Advanced Development for Europe Project, which is associated with the W3C Semantic Web Activity. This document is made available by W3C for discussion only. Publication of this document by W3C does not imply endorsement by W3C, including the Team and Membership.

Comments on this document are welcome and should be sent to the authors or to the public-esw-thes@w3.org list. An archive of this list is available at http://lists.w3.org/Archives/Public/public-esw-thes/.

Contents

1. Introduction   [back to contents]

This document is a guide to accompany the SKOS-Core 1.0 schema, which can be found under the namespace:

http://www.w3.org/2004/02/skos/core#

SKOS-Core 1.0 is an RDF schema for representing thesauri and similar types of knowledge organisation system (KOS). SKOS-Core can be used for:

SKOS stands for Simple Knowledge Organisation Systems. SKOS-Core is intended as a complement to OWL. It does provide a basic framework for building concept schemes, but it does not carry the strictly defined semantics of OWL. Thus it is ideal for representing those types of KOS, such as thesauri, that connot be mapped directly to an OWL ontology. SKOS is also easier to use, and harder to misuse than OWL, providing an ideal entry point for those wishing to use the Semantic Web for knowledge organisation.

SKOS-Core also provides a framework for linking concepts to the words and phrases that are normally used by people to refer to them. This valuable information, once captured, can be used to support a number of tasks, such as automated classification of web documents, and automated multilingual translation of glossaries.

This guide first describes the SKOS meta-model, and introduces the terminology that will be used throughout. There follows a description of the features of the SKOS-Core 1.0 schema, with examples of how to use them. Finally there are guidelines for using SKOS concept schemes for subject-based indexing of web resources.

2. The SKOS Meta-Model   [back to contents]

SKOS-Core allows you to define concepts and concept schemes. A concept is any unit of thought that can be defined or described. A concept scheme is a collection of concepts.

A concept may have any number of attached labels. A label is any word, phrase or symbol that can be used to refer to the concept by people. A concept may have only one preferred label, and any number of alternative labels.

Relationships may be defined between concepts within the same concept scheme. Any such relationship is referred to here as a semantic relation.

Mappings may be defined between concepts from different concept schemes. Any such mapping is referred to here as a semantic mapping.

Figure 1. The SKOS Meta-Model

The SKOS Meta-Model

3. Using SKOS-Core 1.0   [back to contents]

The examples below demonstrate how to use the SKOS-Core 1.0 schema to create a simple concept scheme in the style of a thesaurus.

3.1. Defining a concept scheme   [back to contents]

The first step is to describe the scheme itself. To allow unambiguous reference to your scheme in the future, give it a URI. Then state that it has type skos:ConceptScheme. To describe the meta-properties of the scheme, you can use e.g. Dublin Core, for example:

Example of a concept scheme declaration

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" 
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	xmlns:dc="http://purl.org/dc/elements/1.1/">
	
	<skos:ConceptScheme rdf:about="http:/example.com/thesaurus">
		<dc:title>The SWAD-Europe Example Thesaurus</dc:title>
		<dc:description>An example thesaurus to illustrate the use of the SKOS-Core schema.</dc:description>
		<dc:creator>A J Miles</dc:creator>
	</skos:ConceptScheme>

</rdf:RDF>

3.2. Defining concepts   [back to contents]

For each concept you define, there must be some way for both people and machines to refer to that concept unambiguously in the future. The simplest way to ensure unambiguous global reference is to assign each concept a URI. Then state that the concept has type skos:Concept. For example:

Example of a concept declaration

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0001"/>

</rdf:RDF>

A concept does not have to be a part of any scheme. It may also be a part of more than one scheme. To express that a concept is a part of a scheme, use the skos:inScheme property. For example:

Example of using skos:inScheme

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
	</skos:Concept>

</rdf:RDF>

3.3. Labelling concepts   [back to contents]

A concept may be given only one preferred label, and any number of alternative labels. For example:

Example of concept labelling

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:prefLabel>Bangers and mash</skos:prefLabel>
		<skos:altLabel>Sausage and mash</skos:altLabel>
		<skos:altLabel>Sausage and mashed potato</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
	</skos:Concept>

</rdf:RDF>

To help other people unambiguously refer to your concepts without having to use URIs, it is strongly recommended that no two concepts in your scheme be given the same preferred label.

The alternative labels can be any set of words or phrases, including alternative spelling variants (and mis-spellings), that could be used to refer to the concept. In the traditional thesaurus sense, this set of labels constitutes the 'entry vocabulary' for your concept. It is there primarily to help other people navigate to the right concept. The better the coverage of possible labels for each concept, the more likely another person is to find the concept they want.

3.4. External (non-URI) identifiers for concepts   [back to contents]

If a concept has already been defined outside the scope of the semantic web, it may have been given an unique identifier that is not a URI. To express this property of a concept, use the skos:externalID property. For example:

Example of using skos:externalID

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:externalID>A.01.0001</skos:externalID>
		<skos:prefLabel>Bangers and mash</skos:prefLabel>
		<skos:altLabel>Sausage and mash</skos:altLabel>
		<skos:altLabel>Sausage and mashed potato</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
	</skos:Concept>

</rdf:RDF>

3.5. Adding scope-notes, definitions, contextual examples and depictions   [back to contents]

SKOS-Core 1.0 has three properties to help you establish exactly what each concept is intended to mean. Use skos:scopeNote to attach any piece of text that helps to further elucidate the intended meaning of a concept. Use skos:definition to attach a formal (dictionary-style) definition to a concept. Use skos:example to attach a contextual example of the use of one of the labels (usually the preferred label). For example:

Example of using skos:scopeNote

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0011">
		<skos:prefLabel>Marble</skos:prefLabel>
		<skos:scopeNote>A granular crystalline limestone.</skos:scopeNote>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
	</skos:Concept></rdf:RDF>

Example of using skos:definition

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0012">
		<skos:prefLabel>Liquid crystal</skos:prefLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:definition>A substance which flows like a liquid but has some 
		degree of ordering in the arrangement of its molecules.</skos:definition>
	</skos:Concept>

</rdf:RDF>

Example of using skos:example

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/00013">
		<skos:prefLabel>Medicate</skos:prefLabel>
		<skos:altLabel>Administer medicine</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:example>both infants were heavily medicated to alleviate their symptoms</skos:example>
	</skos:Concept>

</rdf:RDF>

It may also be helpful to attach to a concept images depicting it. This may be done using the foaf:depiction property. For example:

Example of using foaf:depiction

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#" 
	xmlns:foaf="http://xmlns.com/foaf/0.1/"> 

	<foaf:Image rdf:about="http:/example.com/img/bangersandmash.jpg"/>
		
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:prefLabel>Bangers and mash</skos:prefLabel>
		<skos:altLabel>Sausage and mash</skos:altLabel>
		<skos:altLabel>Sausage and mashed potato</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<foaf:depiction rdf:resource="http:/example.com/img/bangersandmash.jpg"/>
	</skos:Concept>

3.6. Using symbols to label concepts   [back to contents]

If it is more appropriate for you to label your concepts with symbols, rather than words or phrases, use the skos:prefSymbol and skos:altSymbol properties. For example:

Example of using symbols to label concepts

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"
	xmlns:foaf="http://xmlns.com/foaf/0.1/"> 

	<foaf:Image rdf:about="http:/example.com/symbols/0001.jpg"/>
	<foaf:Image rdf:about="http:/example.com/symbols/0001a.jpg"/>
	<foaf:Image rdf:about="http:/example.com/symbols/0001b.jpg"/>
		
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:prefSymbol rdf:resource="http:/example.com/symbols/0001.jpg"/>
		<skos:altSymbol rdf:resource="http:/example.com/symbols/0001a.jpg"/>
		<skos:altSymbol rdf:resource="http:/example.com/symbols/0001b.jpg"/>
	</skos:Concept>

</rdf:RDF>

3.7. Basic semantic relations between concepts   [back to contents]

SKOS-Core contains a family of properties for expressing simple relationships between concepts within a concept scheme. The top-level property of this family is the skos:semanticRelation property. Below this are the skos:broader, skos:narrower and skos:related properties.

3.7.1. Broader / Narrower

skos:broader/skos:narrower property hierarchy

skos:semanticRelation                skos:semanticRelation
  |                                    |
  +--skos:broader                      +--skos:narrower

The skos:broader and skos:narrower properties allow you to create a hierarchy of concepts. The properties themselves carry weak semantics - they should be used to express the fact that a concept is in some way more or less general than another. I.e. they imply that the concepts involved may reasonably be arranged into a hierarchy, without being too strict about the exact meaning of the hierarchical relationship. A concept may have any number of broader concepts: poly-hierarchies are allowed.

Example of using skos:broader/skos:narrower

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:prefLabel>Bangers and mash</skos:prefLabel>
		<skos:altLabel>Sausage and mash</skos:altLabel>
		<skos:altLabel>Sausage and mashed potato</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:broader rdf:resource="http:/example.com/Concept/0002"/>
	</skos:Concept>

	<skos:Concept rdf:about="http:/example.com/Concept/0002">
		<skos:prefLabel>English cuisine</skos:prefLabel>
		<skos:altLabel>English dishes</skos:altLabel>
		<skos:altLabel>English food</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:narrower rdf:resource="http:/example.com/Concept/0001"/>
	</skos:Concept>

</rdf:RDF>

3.7.2. Related

skos:related property hierarchy

skos:semanticRelation
  |
  +--skos:related

The skos:related property allows you to create associative links between concepts. Again, this property carries weak semantics - it expresses the fact that two concepts are in some way related, and that the relationship should NOT be used to create a hierarchy. Essentially, this property allows you to create links between branches of a hierarchy of concepts.

Example of using skos:related

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0002">
		<skos:prefLabel>English cuisine</skos:prefLabel>
		<skos:altLabel>English dishes</skos:altLabel>
		<skos:altLabel>English food</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:related rdf:resource="http:/example.com/Concept/0003"/>
	</skos:Concept>

	<skos:Concept rdf:about="http:/example.com/Concept/0003">
		<skos:prefLabel>English pubs, cafes and restaurants</skos:prefLabel>
		<skos:altLabel>English eateries</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:related rdf:resource="http:/example.com/Concept/0002"/>
	</skos:Concept>

</rdf:RDF>

The two examples above give a concept scheme with the following structure:

Diagram of concept structure

Diagram of concept structure

3.8. Multilingual labelling   [back to contents]

A concept may be given labels from different languages. To specify the language of a label, use the language attribute of the RDF literal (see RDF specs). For each language, a concept may have one preferred label and any number of alternative labels. For example:

Example of multilingual labelling

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:prefLabel xml:lang="en">English cuisine</skos:prefLabel>
		<skos:altLabel xml:lang="en">English dishes</skos:altLabel>
		<skos:altLabel xml:lang="en">English food</skos:altLabel>
		<skos:prefLabel xml:lang="fr">Cuisine anglaise</skos:prefLabel>
		<skos:altLabel xml:lang="fr">Plats anglais</skos:altLabel>
		<skos:prefLabel xml:lang="es">Cocina ingles</skos:altLabel>
		<skos:prefLabel xml:lang="it">Cucina inglese</skos:prefLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
	</skos:Concept>

</rdf:RDF>

3.9. More precise semantics   [back to contents]

The SKOS-Core property set is designed to be extensible. The skos:semanticRelation property is the top-level property for all properties linking concepts within the same concept scheme. The first level extension of this property is the three properties already mentioned, skos:broader skos:narrower and skos:related. These may in turn be extended, to create properties that carry more precise semantics. This structure provides a framework for interoperability between concept schemes with different levels of precision in the semantics of their concept-to-concept relations.

A number of extensions are built in to the SKOS-Core 1.0 schema, and these are described below.

3.9.1. BroaderGeneric / NarrowerGeneric

skos:broaderGeneric/skos:narrowerGeneric property hierarchy

skos:semanticRelation                  skos:semanticRelation
  |                                      |
  +--skos:broader                        +--skos:narrower
       |                                      |
       +--skos:broaderGeneric                 +--skos:narrowerGeneric
       
rdfs:subClassOf
  |
  +--skos:broaderGeneric

This extension of the skos:broader/skos:narrower property pair may only be used to specify a class subsumption relationship between two concepts. The semantics of these properties are inherited from the rdfs:subClassOf property.

Example of using skos:broaderGeneric/skos:narrowerGeneric

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0004">
		<skos:prefLabel>English pubs, clubs and bars</skos:prefLabel>
		<skos:altLabel>English drinking establishments</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:narrowerGeneric rdf:resource="http:/example.com/Concept/0005"/>
	</skos:Concept>

	<skos:Concept rdf:about="http:/example.com/Concept/0005">
		<skos:prefLabel>English pubs</skos:prefLabel>
		<skos:altLabel>English public houses</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:broaderGeneric rdf:resource="http:/example.com/Concept/0004"/>
	</skos:Concept>

</rdf:RDF>

3.9.2. BroaderInstantive / NarrowerInstantive

skos:broaderInstantive/skos:narrowerInstantive property hierarchy

skos:semanticRelation                  skos:semanticRelation
  |                                      |
  +--skos:broader                        +--skos:narrower
       |                                      |
       +--skos:broaderInstantive              +--skos:narrowerInstantive
       
rdf:type
  |
  +--skos:broaderInstantive

Use these properties to express an instantive (instance-of) relationship between two concepts. The semantics of these properties are inherited from the rdf:type property.

Example of using skos:broaderInstantive/skos:narrowerInstantive

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0006">
		<skos:prefLabel>The Red Lion, Blewbury</skos:prefLabel>
		<skos:altLabel>The Red Lion pub in Blewbury</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:broaderInstantive rdf:resource="http:/example.com/Concept/0005"/>
	</skos:Concept>

	<skos:Concept rdf:about="http:/example.com/Concept/0005">
		<skos:prefLabel>English pubs</skos:prefLabel>
		<skos:altLabel>English public houses</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:narrowerInstantive rdf:resource="http:/example.com/Concept/0006"/>
	</skos:Concept>

</rdf:RDF>

3.9.3. BroaderPartitive / NarrowerPartitive

skos:broaderPartitive/skos:narrowerPartitive property hierarchy

skos:semanticRelation                  skos:semanticRelation
  |                                      |
  +--skos:broader                        +--skos:narrower
       |                                      |
       +--skos:broaderPartitive               +--skos:narrowerPartitive
       
dcterms:isPartOf                       dcterms:hasPart
  |                                      | 
  +--skos:broaderPartitive               +--skos:narrowerPartitive

Use these properties to express a partitive (part-of) relationship between two concepts, where you would like such relationships to be organised as part of a hierarchical structure. The semantics of these properties are inherited from the dcterms:isPartof/dcterms:hasPart property pair [see ref].

Example of using skos:broaderPartitive/skos:narrowerPartitive

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0007">
		<skos:prefLabel>Oxfordshire county</skos:prefLabel>
		<skos:altLabel>Oxon</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:broaderPartitive rdf:resource="http:/example.com/Concept/0008"/>
	</skos:Concept>

	<skos:Concept rdf:about="http:/example.com/Concept/0008">
		<skos:prefLabel>England</skos:prefLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:narrowerPartitive rdf:resource="http:/example.com/Concept/0007"/>
	</skos:Concept>

</rdf:RDF>

3.9.4. RelatedPartOf / RelatedHasPart

skos:relatedPartOf/skos:relatedHasPart property hierarchy

skos:semanticRelation                  skos:semanticRelation
  |                                      |
  +--skos:related                        +--skos:related
       |                                      |
       +--skos:relatedPartOf                  +--skos:relatedHasPart
       
dcterms:isPartOf                       dcterms:hasPart
  |                                      | 
  +--skos:relatedPartOf                  +--skos:relatedHasPart

Use these properties to express a partitive (part-of) relationship between two concepts, where you would like such relationships to be organised as part of an associative structure. These properties carry the same semantics as the skos:broaderPartitive/skos:narrowerPartitive property pair. These two options exist to provide a framework for interoperability between thesauri that treat the partitive relationship in different ways.

Example of using skos:relatedPartOf/skos:relatedHasPart

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:Concept rdf:about="http:/example.com/Concept/0001">
		<skos:prefLabel>Bangers and mash</skos:prefLabel>
		<skos:altLabel>Sausage and mash</skos:altLabel>
		<skos:altLabel>Sausage and mashed potato</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:relatedHasPart rdf:resource="http:/example.com/Concept/0009"/>
	</skos:Concept>

	<skos:Concept rdf:about="http:/example.com/Concept/0009">
		<skos:prefLabel>Sausages</skos:prefLabel>
		<skos:altLabel>Bangers</skos:altLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:relatedPartOf rdf:resource="http:/example.com/Concept/0001"/>
	</skos:Concept>

</rdf:RDF>

3.10. High-level grouping of concepts   [back to contents]

Many thesauri contain the notion of high-level grouping of concepts. This is done by arranging concepts in hierarchies under special concepts, these special concepts often referred to as 'fundamental facets' or 'fundamental categories'.

To represent the fact that a concept is intended to sit at the top of the concept hierarchy in a scheme, use the skos:TopConcept class.

skos:TopConcept class hierarchy

skos:Concept 
  |
  +--skos:TopConcept

Example of defining a top concept

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:TopConcept rdf:about="http:/example.com/Concept/0010">
		<skos:prefLabel>Materials</skos:prefLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
	</skos:TopConcept>

	<skos:TopConcept rdf:about="http:/example.com/Concept/0012">
		<skos:prefLabel>Processes</skos:prefLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
	</skos:TopConcept>

</rdf:RDF>

Other concepts in the scheme may then be linked to a top concept via any of the standard semantic relation properties, e.g. skos:broader / skos:narrower. For example:

Example of linking concepts to top concepts

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#"> 
	
	<skos:TopConcept rdf:about="http:/example.com/Concept/0010">
		<skos:prefLabel>Materials</skos:prefLabel>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:narrower rdf:resource="http:/example.com/Concept/0011"/>
	</skos:TopConcept>

	<skos:Concept rdf:about="http:/example.com/Concept/0011">
		<skos:prefLabel>Marble</skos:prefLabel>
		<skos:scopeNote>A granular crystalline limestone</skos:scopeNote>
		<skos:inScheme rdf:resource="http:/example.com/thesaurus"/>
		<skos:broader rdf:resource="http:/example.com/Concept/0010"/>
	</skos:Concept>

</rdf:RDF>

Of course, a top concept is not allowed to have any broader concepts.

4. Using SKOS-Core with Dublin Core for subject-based indexing   [back to contents]

One of the main purposes of creating a concept scheme is to use it to help organise a set of resources e.g. a collection of web documents. When metadata about resources is enriched to include some statement about the 'subject' or 'topic' of the resource, and the 'subject' or 'topic' is a concept in some concept scheme, we can use that concept scheme to help search and browse around the collection of resources.

To use SKOS-Core with Dublin Core, make concepts the objects of statements using the dc:subject property. For example:

Example of using SKOS-Core with dc:subject

<rdf:RDF 
	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
	xmlns:skos="http://www.w3.org/2004/02/skos/core#" 
	xmlns:dc="http://purl.org/dc/elements/1.1/">

	<rdf:Description rdf:about="http:/example.com/web/aDocument.html">
		<dc:subject rdf:resource="http:/example.com/Concept/0005"/>
	</rdf:Description>

</rdf:RDF>

Currently we recommend that concepts defined using SKOS-Core are NOT used with the foaf:topic or foaf:primaryTopic properties.

5. Further Reading   [back to contents]

That's the end of this guide. Development of SKOS-Core, and the sister vocabulary SKOS-Mapping (which allows you to assert mappings between concepts from different schemes), is ongoing. Latest news and updates can be found on the website for the SWAD-Europe Thesaurus Activity. You can also track the work via the public mailing list public-esw-thes@w3.org - all contributions are very welcome, and an archive of previous mails is also available.

Other reports and schemas from SWAD-Europe Thesaurus Activity

SWAD-Europe Deliverable 8.2: Review of Existing Work on RDF Thesaurus Schemas.

SWAD-Europe Deliverable 8.3: Multilingual Thesauri.

SWAD-Europe Deliverable 8.4: Inter-Thesaurus Mapping.

SKOS-Mapping: an RDF Schema for Inter-Thesaurus Mapping (pre-release version)

Supporting and related guides

RDF Primer

OWL Web Ontology Language Guide

The FOAF Project

Expressing Qualified Dublin Core in RDF/XML