Word Frequency and Context of Use in the Lexical Diffusion of Phonetically Conditioned Sound ChangeJoan BybeeLexical diffusion refers to the way that a sound change affects the lexicon. If sound change is lexically abrupt, all the words of a language are affected by the sound change at the same rate. If a sound change is lexically gradual, individual words undergo the change at different rates or different times. (...) One early contribution to this debate by Schuchardt (1885) is the observation that high-frequency words are affected by sound change earlier and to a greater extent than low-frequency words. (...) phonetically conditioned changes that affect high-frequency words before low-frequency words are best accounted for in an exemplar model of phonological representation that allows for change to be both phonetically and lexically gradual. (...) a words contexts of use also affect the rate of change. Words that occur more often in the context for change, change more rapidly than those that occur less often in that context. (...) sound changes can also progress more rapidly in high-frequency morphemes. (...) the contexts of use determine the rate at which a word or morpheme undergoes a sound change.1. Regular sound change or lexical diffusion?The hypothesis that sound change is lexically regular seems well supported by thefacts of change. When we observe that two languages or dialects exhibit a phono-logical difference, it is very likely that this difference is regular across all the wordsthat have the appropriate phonetic environment. This observation is fundamental tothe comparative method; the establishment of genetic relations and the reconstruc-tion of protolanguages are based on the premise that sound change affects all wordsequally. Schuchardt (1885) was one of the detractors from this position. When heobserved sound change in progress, he noted that all words did not change at thesame rate and that the differences were not due to dialect mixture, as was oftenclaimed by the Neogrammarians, who supported the regularity position.Labov (1981, 1994) He proposed two types of sound change: regular sound change is gradual, phonetically motivated, without lexical or grammatical conditioning, and not influenced by social awareness, whereas lexical diffusion change, such as the phenomena studied by Wang, is the result of the abrupt substitution of one phoneme for another in words that contain that phoneme (Labov 1994:542). According to Labov, this type of change occurs most often in the late stages of internal change that has been differentiated by lexical and grammatical conditioning (ibid.). Labov went so far as to propose that certain changes, such as the deletion of glides and schwa, would be regular changes, while the deletion of obstruents would show lexical diffusion.(...) even gradual, phonetically conditioned change exhibits gradual lexical diffusion (...)Hooper (1976) identified a lexical diffusion paradox. Reductive sound change tends to affect high-frequency words before low-frequency words, but analogical leveling or regularization tends to affect low-frequency words before high-frequency words.2. Frequency effects on regular sound changeSound changes that are complete can be identified as regular or not, depending upon whether they affected all lexical items existing at the time of the change. Ongoing changes cannot be