RDF Schema RDFS

RDF Schema (RDFS) is extending RDF vocabulary to allow describing taxonomies of classes and properties. It also extends definitions for some of the elements of RDF, for example it sets the domain and range of properties and relates the RDF classes and properties into taxonomies using the RDFS vocabulary.

Let us first illustrate the use of RDFS vocabulary on an example showing taxonomy of classes and properties and usage of range and domain of properties:

    @prefix :     <http://www.example.org/sample.rdfs#> .
    @prefix rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
    @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>.
    
    :Dog      rdfs:subClassOf :Animal.
    :Person   rdfs:subClassOf :Animal.
    
    :hasChild rdfs:range :Animal;
              rdfs:domain :Animal.
    :hasSon   rdfs:subPropertyOf :hasChild.
    
    :Max      a :Dog.
    :Abel     a :Person.
    :Adam     a :Person;
              :hasSon :Abel.

The list of classes defined by RDFS is shown in the table below. All resources can be divided into groups called classes. Classes are also resources, so they are identified by URIs and can be described using properties. The members of a class are instances of classes, which is stated using the rdf:type property. Note that class and a set of instances does not have to be the same. The set of instances is the extension of the class, and two different classes may contain the same set of instances. For example, looking at the RDF example graph, class of people having mailbox mailto:joe.smith@example.org is different from the class of people having homepage http://www.example.org/~joe/, but the extension (i.e., the set of instances) of these classes is the same - it is the resource http://www.example.org/~joe/contact.rdf#joesmith.

Element	Class of	rdfs:subClassOf	rdf:type
rdfs:Resource	all resources	rdfs:Resource	rdfs:Class
rdfs:Class	all classes	rdfs:Resource	rdfs:Class
rdfs:Literal	literal values	rdfs:Resource	rdfs:Class
rdfs:Datatype	datatypes	rdfs:Class	rdfs:Class
rdf:XMLLiteral	XML literal values	rdfs:Literal	rdfs:Datatype
rdf:Property	properties	rdfs:Resource	rdfs:Class
rdf:Statement	statements	rdfs:Resource	rdfs:Class
rdf:List	lists	rdfs:Resource	rdfs:Class
rdfs:Container	containers	rdfs:Resource	rdfs:Class
rdf:Bag	unordered containers	rdfs:Container	rdfs:Class
rdf:Seq	ordered containers	rdfs:Container	rdfs:Class
rdf:Alt	containers of alternatives	rdfs:Container	rdfs:Class
rdfs:Container MembershipProperty	rdf:_1... properties expressing membership	rdf:Property	rdfs:Class

RDFS classes

In RDFS a class may be an instance of a class. All resources are instances of the class rdfs:Resource. All classes are instances of rdfs:Class and subclasses of rdfs:Resource. All literals are instances of rdfs:Literal. All properties are instances of rdf:Property. The rdfs:subClassOf (subclass-of relation) and rdf:type (instance-of relation) for all RDFS classes are shown in the the table above.

Element	Relates	rdfs:domain	rdfs:range
rdfs:range	restricts subjects	rdf:Property	rdfs:Class
rdfs:domain	restricts objects	rdf:Property	rdfs:Class
rdf:type	instance of	rdfs:Resource	rdfs:Class
rdfs:subClassOf	subclass of	rdfs:Class	rdfs:Class
rdfs:subPropertyOf	subproperty of	rdf:Property	rdf:Property
rdfs:label	human readable label	rdfs:Resource	rdfs:Literal
rdfs:comment	human readable comment	rdfs:Resource	rdfs:Literal
rdfs:member	container membership	rdfs:Resource	rdfs:Resource
rdf:first	first element	rdf:List	rdfs:Resource
rdf:rest	rest of list	rdf:List	rdf:List
rdf:_1, rdf:_2, ...	container membership	rdfs:Container	rdfs:Resource
rdfs:seeAlso	further information	rdfs:Resource	rdfs:Resource
rdfs:isDefinedBy	definition	rdfs:Resource	rdfs:Resource
rdf:value	for structured values	rdfs:Resource	rdfs:Resource
rdf:object	object of statement	rdf:Statement	rdfs:Resource
rdf:predicate	predicate of statement	rdf:Statement	rdfs:Resource
rdf:subject	subject of of statement	rdf:Statement	rdfs:Resource

RDFS properties (all are instances of rdf:Property)

Properties in RDFS are relations between subjects and objects in RDF triples, i.e., predicates. The properties as defined by RDFS are listed in the table with RDFS properties. All properties may have defined domain and range. Domain of a property states that any resource that has given property is an instance of the class. Range of a property states that the values of a property are instances of the class. If multiple classes are defined as the domain and range then the intersection of these classes is used. Ranges and domains for RDFS properties are summarized in the table above. An example stating that the domain of hasSon property is Person and that the domain of the same property is Man follows:

    @prefix :        <http://www.example.org/sample.rdfs#> .
    @prefix rdfs:    <http://www.w3.org/2000/01/rdf-schema#>.
    
    :hasSon rdfs:domain :Person;
            rdfs:range  :Man.

The taxonomy of classes is formed by property rdfs:subClassOf, taxonomy of properties is formed by property rdfs:subPropertyOf.

RDFS also defines one instance that is neither class nor property. This instance denotes empty list: rdfs:nil, see the table with the RDFS instance.

Element	Meaning	rdfs:type
rdfs:nil	empty list	rdf:List

RDFS instance