Combining studies of living communities with examinations of historical records of completed changes, this book traces the general principles of chain shifting within and across vowel systems, as well as the principles that govern mergers and splits. Labov uses evidence from sociolinguistics and dialect geography to provide responses to the controversies initiated by the neo-grammarians. Though lexical diffusion can be located in changes of membership in abstract categories such as "long" and "short", the main agent of linguistic change, Labov argues, is regular sound change that proceeds without regard to the preservation of meaning. Change in sound can lead to confusion and the disruption of dialect systems. The findings presented here on the asymmetry of production and perception explain the historical continuity of word classes when the semantic function of contrast is suspended.
William Labov was an American linguist widely regarded as the founder of the discipline of variationist sociolinguistics. He has been described as "an enormously original and influential figure who has created much of the methodology" of sociolinguistics, and "one of the most influential linguists of the 20th and 21st centuries". Labov was a professor in the linguistics department of the University of Pennsylvania in Philadelphia and pursued research in sociolinguistics, language change, and dialectology. He retired in 2015 but continued to publish research until his death in 2024.