The fundamental reason for database management systems is the support of applications using shared overlapping sets of data through external views of an underlying conceptual schema. The definition of such a schema for expert systems (ESs) is a topic of interest to database researchers both as a proof that ESs can be supported and as an exploration of the process of view integration. A shared conceptual schema has been constructed for four widely known medical ESs, and can be mapped to the knowledge base of each using seven operations: COMPOSE, FILTER, FOLD, UNFOLD, CONTRACT, RENAME, and CONSTRAIN. The schema and mappings show features the ESs share, such as rules and patterns, and features that distinguish them, such as a unique form of diagnosis. The schema's content shows a lack of structural, temporal, or strategic information and illustrates how the types and relationships of shell-like systems are hidden in instances and must be interpreted by program code.