This article reports on research carried out in the Fens in Eastern England, a region noted in the dialectological literature as the site of a number of important phonological transitions, most notably [] and [a – a:], which separate northern and southern varieties of British English. Recordings of 81 speakers from across the Fens were analyzed for the use of (ai), a particularly salient local variable. A “Canadian Raising” type of allophonic variation was found in the central Fenland: speakers in this area used raised onsets of (ai) before voiceless consonants but open onsets before voiced consonants, morpheme boundaries, and //. The article weighs a number of possible explanations for the emergence of this variation in the Fens. Based on compelling evidence from the demographic history of the area, it supports a view that such an allophonic distribution, previously thought not to be found in Britain, emerged as the result of dialect contact. The sociolinguistic process of koinéization that is commonly associated with post-contact speech communities (Trudgill 1986) is held responsible for the focusing of this allophonic variation from the input dialects of an initially mixed variety. The article concludes by suggesting a socially based explanatory model to account for the way that speakers implement processes of focusing and koinéization in areas of dialect contact. [English, dialects, contact, koiné, geographical linguistics, social networks, structuration theory)