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Ontologies are critical for applications that want to search across or merge information from diverse communities. Although XML DTDs and XML Schemas are sufficient for exchanging data between parties who have agreed to definitions beforehand, their lack of semantics prevent machines from reliably performing this task given new XML vocabularies. The same term may be used with (sometimes subtle) different meaning in different contexts, and different terms may be used for items that have the same meaning.
RDF and RDF Schema begin to approach this problem by allowing simple semantics to be associated with identifiers. With RDF Schema, one can define classes that may have multiple subclasses and super classes, and can define properties, which may have sub properties, domains, and ranges. In this sense, RDF Schema is a simple ontology language. However, in order to achieve interoperation between numerous, autonomously developed and managed schemas, richer semantics are needed. For example, RDF Schema cannot specify that the Person and Car classes are disjoint, or that a string quartet has exactly four musicians as members. --http://www.w3.org/TR/webont-req/
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