Recent investigations of the history of Chinese have given new support to the view that sound change diffuses gradually across the lexicon. Yet instrumental studies of sound change in progress support the Neogrammarian position that change affects all words that include the sound according to their phonetic environment. The paradox can be resolved by distinguishing abstract phonological change from change in low-level output rules. Both types of rules can be observed in recent studies of sound change in progress in Philadelphia: the lexical split of short a shows lexical diffusion in progress, while raising, lowering, fronting, and backing rules show Neogrammarian regularity. A review of the literature on completed changes and other changes in progress tends to support the relevance of a hierarchy of abstractness in determining the nature of the transition from one stage to the other.
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