English a-tensing has received numerous treatments in the phonological and sociolinguistic literature, but the question of why it occurs (i) at all and (ii) in seemingly unnatural disjunctive phonological environments has not been settled. This paper presents a novel phonetic enhancement account of a-tensing in Philadelphia, New York City and Belfast English. I propose that a-tensing is best understood as an allophonic process which facilitates the perceptual identity and articulatory ease of nasality, voicing and/or segment duration in the following consonant. This approach unifies the apparently unnatural phonological environments in which the two a variants surface and predicts the attested dialectal patterns. A synchronic account of a-tensing also provides an explanation for the suprasegmental and morphological factors that condition the process.