Neogrammarian regularity hypothesis represents the phonetically gradual, lexically abrupt sound change. Lexical diffusion, on the contrary, supposes the phonetically abrupt, lexically gradual sound change. According to Labov, neogrammarian change type and lexical diffusion change type coexist in sound change, so these two seemingly oppositional theories, in fact, mutually complementary. In this respect, lexical diffusion contributed to the study of sound change by fundamentally broadening its horizons.