Knowledge-based systems have become ubiquitous in recent years. The World-Wide Web consortium is developing the Resource Description Framework (RDF) – a system for annotating even Web pages with knowledge elements. Knowledge-base developers need to be able to share and reuse knowledge bases that they build. Therefore, interoperability among different knowledge-representation systems is essential. The Open Knowledge-Base Connectivity protocol (OKBC) is a common query and construction interface for frame-based systems that facilitates this interoperability. Protege-2000 is an OKBC-compatible knowledge-base?editing environment developed in our laboratory. Protege-2000 has an easy-to-use and configurable interface. We describe its OKBC-compatible knowledge model that makes the import and export of knowledge bases from and to other knowledge-base servers easy. We discuss how the requirements of being usable and configurable knowledge-acquisition tool affected our decisions in the knowledge-model design. Protege-2000 also has a flexible metaclass architecture which provides configurable templates for new classes in the knowledge base. The use of metaclasses makes Protege-2000 easily extensible and enables its use with other knowledge models. For example, we demonstrate that we can resolve many of the differences between the knowledge models of Protege-2000 and RDF by defining a new metaclass set. Resolving the differences between the knowledge models in declarative way enables easy adaptation of Protege-2000 as an editor for other knowledge-representation systems.
