The study of sound change in progress in Philadelphia has been facilitated by the application of forced alignment and automatic vowel measurement to a large corpus of neighborhood studies, including 379 speakers with dates of birth from 1888 to 1991. Two of the sound changes active in the 1970s show a linear pattern of incrementation in succeeding decades. The fronting of back upgliding vowels /aw/ and /ow/ shows a reversal in the direction of change, beginning with those born after 1940. The study also finds a general withdrawal from two salient features of local phonology, tense /æh/ and /oh/, led by those with higher education. Younger speakers with higher education have also reorganized the traditional Philadelphia tense/lax split of short-a to form a nasal system with tensing before all and only nasal consonants. The development of the Philadelphia vowel system can be understood in the geographic context of neighboring dialects. Features in common with North and North Midland dialects have accelerated in use while features in common with South Midland and Southern dialects have been reversed in favor of Northern patterns. The microevolution of a linguistic system can be seen here as subject to phonological generalizations but driven by social evaluation as features rise in level of salience for members of the speech community.