2.1. The soks:Concept Class

The simplest way to define a concept in RDF is give it an explicit URI, and type it as a soks:Concept. For example ...

<soks:Concept rdf:about="http://www.w3c.rl.ac.uk/examples/thesaurus#food"/>

2.2. Labelling Concepts

A concept may given a label using the rdfs:label property. For example ...

<soks:Concept rdf:about="http://www.w3c.rl.ac.uk/examples/thesaurus#sausages">
	<rdfs:label>Sausages</rdfs:label>
</soks:Concept>

A label is considered to be a lexical representation of a concept.

2.3. Preferred Labels

A concept may have a number of alternative labels. To express the fact that one of these labels is the preferred alternative, use the soks:prefLabel property. For example ...

<soks:Concept rdf:about="http://www.w3c.rl.ac.uk/examples/thesaurus#sausages">
	<soks:prefLabel>Sausages</soks:prefLabel>
	<rdfs:label>Bangers</rdfs:label>
</soks:Concept>

The soks:prefLabel property is a sub-property of rdfs:label.

2.4. Descriptors

In well defined conceptual schemes like most thesauri, concepts are given labels that uniquely identify them within the schema, and disambiguate them from other concepts. Such a label is called a descriptor. To define concepts with descriptors, use the soks:descriptor property. For example ...

<soks:Concept rdf:about="http://www.w3c.rl.ac.uk/examples/thesaurus#oranges">
	<soks:descriptor>Oranges (fruit)</soks:descriptor>
	<rdfs:label>Oranges</rdfs:label>
</soks:Concept>

The soks:descriptor proeprty is a sub-property of soks:prefLabel. Therefore a concept cannot have both a descriptor and a prefLabel.

2.5. External Identifiers

In many conceptual schemes the concepts are given independent non-lexical identifiers. An example of such an identifier would be the classification codes used in the DDC. To make an external identifier a part of the concept definition in RDF, use the soks:externalID property. For example ...

<soks:Concept rdf:about="http://www.w3c.rl.ac.uk/examples/thesaurus#oranges">
	<soks:descriptor>Oranges (fruit)</soks:descriptor>
	<rdfs:label>Oranges</rdfs:label>
	<soks:externalID>A.00.01</soks:externalID>
</soks:Concept>

3. Defining Concepts Without URIs

In the above examples all concepts had been given explicit URIs. However, it is possible to refer to concepts without using URIs. The method is called reference by description. Using this method, a concept is left as a blank node, and it is referred to by its properties. If a property or combination of properties uniquely identifies a concept then this method will work. This is similar to the idea of defining a field or combination of fields as a key in databases. In ontologies, such a property is called an inverse functional property.

3.1. Using rdfs:isDefinedBy

The soks:descriptor property uniquely identifies a concept within a conceptual scheme. So a combination of this property, plus a property that identifies the scheme of which this concept is a member, provides a globally unique reference for this concept. To express the membership of a concept within a conceptual scheme, use the rdfs:isDefinedBy property. For example ...

<soks:Concept>
	<soks:descriptor>English food</soks:descriptor>
	<rdfs:label>English cuisine</rdfs:label>
	<rdfs:isDefinedBy rdf:resource="http://www.w3c.rl.ac.uk/examples/thesaurus"/>
</soks:Concept>

... is a perfectly acceptable definition of a concept in RDF.

The soks:externalID property also uniquely identifies a concept within a conceptual scheme, and so can also be used in combination with rdfs:isDefinedBy to constitute a globally unique reference.

Therefore, in order to use reference by description, a concept must be defined with either a soks:descriptor or a soks:externalID or both.

3.2. Using a Subclass of soks:Concept

An alternative to using the rdfs:isDefinedBy property to express membership, you can create a subclass of the soks:Concept class. All concepts typed with your new subclass will be understood to be members of your conceptual scheme. So for example for the SWAD example thesaurus I could do the following ...

<rdfs:Class rdf:about="http://www.w3c.rl.ac.uk/examples/thesaurus#Concept">
	<rdfs:subClassOf rdf:resource="http://www.w3c.rl.ac.uk/2003/11/21-skos-core#Concept"/>
</rdfs:Class>

... which would allow me to define concepts as in the following example ...

<swad:Concept>
	<soks:descriptor>English food</soks:descriptor>
	<rdfs:label>English cuisine</rdfs:label>
</swad:Concept>

The above also is a perfectly acceptable globally unique definition of a concept in RDF.

4. Defining Relations Between Concepts

4.1. Broader, Narrower and Related

The SKOS vocabulary includes the following properties for defining relationships between concepts: soks:broader, soks:narrower, soks:related. The properties soks:broader and soks:narrower should be considered to be each other's inverse. These two properties can be used to define a hierarchical structure within a conceptual scheme. These properties carry no more specific semantics than that one concept is in some sense more general than another. The soks:related property can be used to define an associative structure. This property implies that one concept is in some way associated with another.

An example of using these properties, for concepts defined with explicit URIs ...

<soks:Concept rdf:about="http://www.w3c.rl.ac.uk/examples/thesaurus#002">
	<soks:descriptor>Java programming language<soks:descriptor>
	<rdfs:label>Java</rdfs:label>
	<soks:broader rdf:resource="http://www.w3c.rl.ac.uk/examples/thesaurus#007"/>
	<soks:narrower rdf:resource="http://www.w3c.rl.ac.uk/examples/thesaurus#011"/>
	<soks:related rdf:resource="http://www.w3c.rl.ac.uk/examples/thesaurus#008"/>
</soks:Concept>

Another example, for concepts defined by their properties ...

<swad:Concept>
	<soks:descriptor>Cod (fish)<soks:descriptor>
	<rdfs:label>Cod</rdfs:label>
	<soks:broader>
		<swad:Concept>
			<soks:descriptor>Salt-water fish</soks:descriptor>
		</swad:Concept>
	</soks:broader>
	<soks:related>
		<swad:Concept>
			<soks:descriptor>Fish and chips (cuisine)</soks:descriptor>
		</swad:Concept>
	</soks:related>
	<soks:narrower>
		<swad:Concept>
			<soks:descriptor>North atlantic cod</soks:descriptor>
		</swad:Concept>
	</soks:narrower>
</swad:Concept>

4.2. More Precise Semantics

Many thesauri employ relations that are semantically more precise than simply 'broader'. To make more precise statements using the SKOS vocabulary, you may use the following properties: soks:broaderGeneric, soks:broaderInstantive, and soks:broaderPartitive. These all have a narrower inverse. These properties are all sub-properties of soks:broader, so an application that does not recognise these more precise relationships can at least treat them as broader.

Example of using precise realtionships

The soks:broaderGeneric property is semantically equivalent to rdfs:subClassOf, so these two properties may be used interchangeably. The soks:broaderInstantive property is semantically equivalent to rdf:type, so these two properties may be used in each others's stead.

4.3. Customisation and Extension

All the properties linking concepts to each other are part of a hierarchy derived from the super-property soks:semanticRelation. It is possible to customise and extend the SKOS vocabulary by defining your own semantic relations. By defining them as sub-properties ot soks:semanticRelation you provide some information about how these properties shold be treated by other applications. By defining them as sub-properties of soks:broader or soks:related you provide even more information to other applications about their intended meaning. Therefore we recommend that when defining custom semantic relations, you use sub-property statements to link these to the semantic relations already present in the SKOS vocabulary.

*********Image: semanticrelation property hierarchy**********************

5. Defining Facets

5.1. The soks:Facet Class

To specify that a concept by treated as a facet head, type the concept as a soks:Facet. The soks:Facet class is a subclass of soks:Concept. So for example ...

Example of facet definition

5.2. Expressing Facet Membership

To express facet membership for a concept, use the soks:inFacet property. So for example ...

Example of facet membership

The soks:inFacet property is a sub-property of soks:broader. Therefore, because also soks:Facet is a subclass of soks:Concept, applications that do not understand faceted structures can treat them as part of a simpler concept hierarchy.

***************Image: facet subclass and infacet subproperty*************

6. Scope Notes and Other Comments

The following properties are part of the SKOS vocabulary, and allow scope-notes of various types to be added to concepts: ********add scope note properties*********. They may be used as in the following example ...

Example of scope note usage

7. Using Concepts for Subject Indexing and Classification

We recommend the use of qualified dublin core. If the Concepts have been defined with explicit URIs, then a subject declaration would look like ...

<rdf:Description rdf:about="http://www.w3c.rl.ac.uk/examples/web/page_java.html">
	<dc:subject rdf:resource="http://www.w3c.rl.ac.uk/examples/thesaurus#002"/>
</rdf:Description>

If the concepts have been defined by their properties, we use reference by description as follows ...

<rdf:Description rdf:about="http://www.w3c.rl.ac.uk/examples/web/page_cod.html">
	<dc:subject>
		<soks:Concept>
			<soks:descriptor>Cod (fish)</soks:descriptor>
			<rdfs:isDefinedBy rdf:resource"http://www.w3c.rl.ac.uk/examples/thesaurus"/>
		</soks:Concept>
	</dc:subject>
</rdf:Description>

Because the domain of the soks:descriptor and soks:externalID properties is restricted to Concepts, it is also perfectly acceptable to use the shortened syntax with rdf:parseType="resource" ...

<rdf:Description rdf:about="http://www.w3c.rl.ac.uk/examples/web/page_cod.html">
	<dc:subject rdf:parseType="resource">
		<soks:descriptor>Cod (fish)</soks:descriptor>
		<rdfs:isDefinedBy rdf:resource"http://www.w3c.rl.ac.uk/examples/thesaurus"/>
	</dc:subject>
</rdf:Description>

Or if a subclass of soks:Concept has been defined, you may use this as part of the concept description, for example ...

<rdf:Description rdf:about="http://www.w3c.rl.ac.uk/examples/web/page_cod.html">
	<dc:subject>
		<swad:Concept>
			<soks:descriptor>Cod (fish)</soks:descriptor>
		</swad:Concept>
	</dc:subject>
</rdf:Description>

Summary