The Unified Modeling Language is extensible, and so can be regarded as a family of languages. Implicitly or explicitly, any particular UML model should be accompanied by a definition of the particular UML family member used for the model. The definition should cover syntactic and semantic issues. This paper proposes a mechanism for associating models with such definitions. Any particular definition would form what we call a preface. The name is intended to suggest that the definition of a particular UML family member must conceptually come before any model built using that family member. A preface would be large, and should be organized using packages. This would allow large amounts of sharing between different prefaces. The paper proposes that prefaces should have an axiomatic style of semantics, though not necessarily fully formal, and it offers a general approach to semantics that would reduce problems of inconsistency within a large preface, based on the idea of general cases and special cases.