The RDF Schema Specification Revisited Wolfgang Nejdl and Martin Wolpers and Christian Capelle Institut für Technische Informatik Rechnergestützte Wissensverarbeitung Universität Hannover Appelstraße 4, 30167 Hannover {nejdl,wolpers,capelle}@kbs.uni-hannover.de 1 Abstract In this paper, we will discuss the proposed recommendation (March 1999) for an RDF Schema Specification and compare the approach taken in this specification to conventional meta data models. We will point out several pecularities in the RDF schema specification compared to more conventional approaches, which are basically due to the dual role of properties such as subclass and domain both as primitive constructs used in the definition of the RDF schema specification and as specific instances of RDF properties. We then discuss an approach for specifying RDF schemas, which distinguishes between a set of specific properties, which are needed in the RDF meta model itself, and conventional properties. Finally we show how we use this modified RDF meta model in a structured hypertext system (the KBS Hyperbook System), which uses RDF annotations not only for informational purposes, but also for structuring hypertext according to the semantic relationships recorded by these RDF annotations. 2 Introduction Semantic annotations and metadata are seen as a crucial technique for transforming the World Wide Web from a huge set of amorphous pages interlinked with each other to a semantic web, which uses semantic annotations in order to give meaning to these pages and their relationships [Lass 1998]. This is in line with the development in other disciplines such as database and software engineering, which use semantic data models and schemas to define and give meaning to data and parts of programs (e.g.[CJB 1998, Guar 1998]). The standard for initiating this transformation is the RDF (Resource Description Framework) Standard [W3C 1998], which has been defined in a set of two documents defining the syntax and semantics of RDF annotations and of RDF schemas. Using RDF, WWW resources are annotated with semantical information, which uses underlying conceptual models (schemas) to define the classes and properties used for these semantic annotations. The shared use of such a schema makes it possible for a group of people to use semantic annotations, which are comparable with each other, because they use the same set of concepts and properties to describe their WWW pages. In this paper, we will discuss the proposed recommendation (March 1999) for an RDF Schema Specification [W3C 1999b]. We will point out several pecularities in the RDF