This paper examines the phonemic status of the vowels in the lexical sets of TRAP, BATH, PALM and START in the English of the southwest of England. In the reference accent RP there are two phonemes; a short front vowel in TRAP and a long back vowel in BATH, START and PALM. In the southwest of England however, some have previously described this contrast as “absent or variable” or “doubtful” (Wells 1982, Hughes, Trudgill and Watt 2005) while others consider there to be a two phoneme system, akin to RP, but differing phonetically (Wakelin 1986). This paper elucidates the status of these vowels using sociolinguistic interview data from 40 speakers in four age groups from locations across Dorset, a representative dialect of the southwest of England. An acoustic analysis of the quality and crucially the length of 3800 vowel tokens reveals that a phonemic split is in progress in apparent time with one phoneme becoming two. The split and subsequent phonetic changes are occurring in a non-uniform way: the backing of the ‘BATH lexical set’ appears to be proceeding via lexical diffusion whereas the backing of START, appears to be a regular ‘neogrammarian’ sound change. The analysis also revealed that the ‘short /a/’ phoneme could be realised long before many following environments. Common environments and constraints on lengthened /a/ cross-dialectally, for example, the shared preference for a lengthened /a/ in closed syllables raise the possibility that the tensing and raising of short a in New York City English (Labov 1994, Trager 1940) and Philadelphia English (Ferguson 1972) and the occurrence of lengthened /a/ in Australia (Blake 1985) may have their origins in the southwest of England.