Metadata and the Semantic Web - Web Standards: Mastering HTML5, CSS3, and XML - page 269

• Union of two or more class descriptions. For example, the union of the previous example

contains the individuals Manual , Guide , Prospectus , Specs , and Overview (if they are all
different).

• Complement of a class description. The class extension contains exactly those individuals

that do not belong to the class extension of the class description that forms the object of the
statement. The complement can be described by the owl:complementOf property.
Class descriptions can be combined into class axioms . Class hierarchy can be expressed by subclass axioms
(Listing 7-84).

Listing 7-84. Class Hierarchy in OWL

SubClassOf( :Slide :Smartphone )

The equivalence of two classes express that the individuals contained by them are identical. Listing 7-85 shows
an example.

Listing 7-85. Equivalent Classes in OWL

EquivalentClasses( :Virtualkeyboard :Softquerty )

Although individuals can be members of several classes in general, in many cases memberships are exclusive.
For example, a smartphone belongs to either the bar or the slide form factor. This class disjointness can be expressed
as shown in Listing 7-86.

Listing 7-86. Class Disjointness in OWL

DisjointClasses( :Bar :Slide )

Simple Knowledge Organization System (SKOS)
Simple Knowledge Organization System (SKOS) is a W3C recommendation for representing taxonomies, thesauri,
classification schemes, subject-heading systems, and structured controlled vocabularies. Being one of the most
frequently implemented Semantic Web standards in industrial applications, SKOS is built upon RDF and RDFS to
enable easy publication of controlled vocabularies as linked data. RDF provides interoperability, consistency, and
integrity, and allows knowledge organization systems to be used in distributed, decentralized metadata applications
where metadata are retrieved from multiple resources.
The SKOS standard defines the SKOS data model as an OWL Full ontology [159]. The elements of the SKOS data
model are OWL classes and properties with individual URIs that form the SKOS vocabulary.

Combining Metadata
Schemas are often combined on the Semantic Web in order to apply all those specific vocabularies that are designed
to express metadata on the topic of the web pages being described. The longest lists of namespaces occur in web
documents that provide a significant amount of additional metadata, most commonly, in (X)HTML+RDFa. Listing 7-87
shows an example.