as more generalized constructions as storage and processing units (Bybee and Scheibman 1999; Bybee 2000).The view of sound change as affecting sounds in words according to their context of use allows us to understand why most phonological alternations occur at the word level: alternations can only be established in cases in which the conditioning environment is present in the storage and processing unit. Words or other units that occur in alternating environments that are not part of the stored unit will not have variants but rather will resolve any variation in favor of one form or the other. This proposal also allows us to make interesting predictions about the development of liaison or sandhi phenomena. Conventionalized alternations across traditional word boundaries indicate that at least one alternate is part of a larger stored unit. Thus such liaison alternations can be used to study the nature and size of storage units. Finally, the view of sound change as affecting sounds in words provides an account of the different effects of alternating environments inside of words and across word boundaries.The second major aspect of the model presented in this essay is that sound change has an immediate and permanent effect on stored representations. This view contrasts with the generative and structural view that underlying representations remain fixed and sound change is rule additionnothing more than a change in the phonological component. The evidence that sound change has an immediate effect on the lexicon is that words change gradually and at different rates according to their token frequency, even while a rule is still variable. The evidence that such change is permanent is the fact that old underlying forms never resurface, even when the phonological rule becomes unproductive (see Cole and Hualde 1998 for more evidence on this point). Instead the progress of a change is inexorably unidirectional in both a phonetic and a morpho-lexical sense. In the phonetic sense, we see the unidirectionality in chains of reduction and assimilation changes, such as those shownin (8), where one change builds on the other and continues its direction:(8)tdshk kj cIf stored items are changed gradually and the motivation for increased automation remains fairly constant, then the continuous nature and strict directionality of such changes is predicted. If sound change were rule addition, there would be no explanation for why, for example, after adding the rule d / V_V, a language wouldgo on to add the rule d / V_V.Inexorable unidirectionality is also apparent in the morphologization and lexicalization of the results of sound change. While no one disputes that morphologization eventually takes place, I have shown here and elsewhere that involvement with the lexicon and grammar occurs very early (Hooper 1976a, 1981; Bybee 2000). Examples given earlier are the word frequency effect in lexical diffusion, the lower rate of deletion of morphemic /t/ and /d/ in American English, the appearance of aspiration for earlier /s/ before a vowel at the end of a word in Cuban Spanish, and the examples described as the Timberlake Effect. Once involved with the lexicon and morphology, alternations become more and