from the conventionalized alternations that can eventually arise from such sound change. By alternation I mean that a word or morpheme has two or more variants that are not phonetically continuous or variable but rather constitute discrete alternants conditioned by specific phonological, grammatical, or lexical contexts. An alternation, then, roughly corresponds to the level of variation generated by a classical phonological rule. By studying the conditions under which such alternations are conventionalized and the conditions under which they are not, we learn something about how variants of words and morphemes are organized in memory.The study of alternating environments in cases of sound change in progress reveals that the outcome differs according to whether it is a word or a morpheme that is in the alternating environment. When the same morpheme is in an alternating environment in different words, a change is retarded even in the conditioning environment (the Timberlake Effect; see below), but an alternation can eventually arise. When the alternates are in two forms of the same word, alternations arise only under special conditions, but ordinarily only one alternate survives. I will argue that the differential behavior of morphemes and words with respect to sound change in progress provides strong evidence for the stored representation of words and frequent phrases.(...)Ordinarily, alternations do not develop where the conditioning is across a word boundary. This fact gives rise to the notion of word-level phonologythat is, the fact that most alternations occur within words. The explanation being investigated here is that ordinarily there is only one stored representation for each word. Where variation arises during change in progress, the variation is resolved in terms of one variant or the other. The exceptions to this arise only in the case of frequently used phrases, to which I will return shortly.First let us consider how variation at the word level is represented and how cases of sound change in an alternating environment would eventually be resolved. In the exemplar model described earlier, the representation of a word is a cluster of actually occurring tokens, with more frequent tokens accumulating greater weight or strength. Thus each word has its own range of variation dependent upon its frequency and the contexts in which it is used. When little or no sound change is affecting a word, the range of variation in the tokens may be small and relatively stable. During change, however, the range of variation increases and the center of the cluster gradually shifts.When the same word occurs in both an environment that conditions a change and a non-conditioning environment, as in the Spanish s-aspiration case, the cluster for a word may divide into two (or more) subclusters, each one with a strong center of high-frequency tokens. In this case, each subcluster is associated with one environment -- the word-final [s] tokens with the environment before a vowel and the word-final [h] tokens with the environment before a C. It appears that such a situation is unstable when the environment is not also part of the representation, because it tends to be resolved in favor of one