than are low-frequency regular verbs. In order for a frequency effect to accrue to a word, that word must exist in memory storage. Since multi-morphemic words evince frequency effects, they must be stored in the lexicon. (...)5. The effect of frequency of use in contextGiven that the exemplar model tracks tokens of use and the exemplar cluster changes according to the phonetic shape of these tokens, it follows that if the context of use affects the phonetic shape of a word, the exemplar cluster will change accordingly. The effect of context can be best exemplified in changes that take place around word or morpheme boundaries, where the segment affected by the change is sometimes in the context for the change and sometimes not. Timberlake (1978) called this an alternating environment. Since the exemplar model registers phonetic tokens, the probabilities inherent in the alternating environment affect the shape of the exemplar cluster. (...) The exemplar cluster, then, appears to reorganize itself, with the stronger exemplars being more frequently chosen for use than the less frequent ones despite the context.Thus along with the general measure of frequency of use, the relative frequency of the immediate linguistic context of use can also affect the lexical diffusion of a sound change. Even holding frequency constant, a word that occurs more often in the right context for a change will undergo the change more rapidly than a word that occurs less often in the conditioning context.(...)8. Consequences for a usage-based theory The study of the diffusion of sound change in the lexicon contributes to a better understanding of the nature and causes of sound change. Changes that affect high-frequency words first are a result of the automation of production, the normal overlap and reduction of articulatory gestures that comes with fluency (Browman and Goldstein 1992; Mowrey and Pagliuca 1995). The strong directionality of such changes indicates that they are not the result of random variation, but that they stem from reduction processes resulting from repetition and the normal automation of motor activity. If a sound change does not proceed from the most frequent to the least frequent words, then we should seek its explanation in some other mechanisms of change.Moreover, I have proposed a model in which variation and change are not external to the lexicon and grammar but inherent to it (Pierrehumbert 1994). Sound change is not rule additionsomething that happens at a superficial level without any effect on the deeper reaches of grammar. Rather, lexical representations are affected from the very beginnings of the change. Indeed, they supply an ongoing record of the change since they track the details of the phonetic tokens experienced. Further evidence for sound change having an immediate impact on representation is the fact that sound changes are never reversed or undone (Cole and Hualde 1998; Bybee 2001). The morphological structure of words also plays a role from the initial stages of a change, but less because morphemes have some special status with respect to change and more because of the contexts in which they appear. Alternating contexts retard change, while