The Resource Description Framework Schema (RDFS) is an extension to RDF that describes how to define RDF vocabularies using RDF itself [21]. It defines, among other things, two important properties, rdfs:subClassOf and rdfs:subPropertyOf, where “rdfs:” is an abbreviation for http://www.w3.org/2000/01/rdf-schema#. The semantics of the property rdfs:subClassOf is that it denotes a specialization of a more general class. If a resource is of type T and T is a rdfs:subClassOf class C, then the resource must also be of type C. In analogy, rdfs:subPropertyOf denotes a specialization of a more general property. If we return to our earlier example, we can use RDFS to extend the metadata set without breaking the backward compatibility.