We present in this paper an extensible metalevel system, called M1991, in which the syntax and the semantics of data models, schemas, and databases can be uniformly described. We show with examples how to derive in M(DM) the semantic equivalence (or lack thereof) of symbols across different reresentation systems. We argue that, because of the inherent incompleteness of legacy databases, semantic equivalence must in general be ascertained by additional a posteriori assertions that are external to the representation under consideration. © 1991, ACM. All rights reserved.