Formal specifications are difficult to understand for a number of reasons. When the developer of a large specification explains it to another person, he typically includes informatlon in his explanation that is is not present, even implicitly. in the specification itself. One useful form of information presents the specification in terms of an evolution from simpler specificattons. TypIcally a specification was actually produced by a series of evolutionary steps reflected in the explanation. This paper suggests three dimensions of evolution that can be used to structure specification developments: structural granularity, temporal granularity. and coverage. Their use in a particular example is demonstrated.