This paper addresses the metrics of expert system reusability through practical experience obtained in developing and using a previously implemented system called INDEX. It presents a principled approach in defining expert system component granularity, transparency, and a specification methodology in identifying and building reusable expert system skeletons and components to allow them to be used in most conventional software environments. The adoption of such an approach has led to the production of reusable expert system components for different expert system development projects. The preliminary experience in reusing INDEX facilities has been presented. Two applications are chosen to show the effectiveness of such an approach.