The source dialects in Britain are critical to disentangling the history and development of varieties in North America and elsewhere. One feature which appears to provide a critical diagnostic, particularly for situating dialects geographically in Britain, is negative (neg) vs. auxiliary (aux) contraction with be, have and will. Use of aux contraction is said to be more prevalent in northern varieties. Using the comparative method and quantitative methodology, this paper provides a quantitative analysis of this feature in eight British communities, two in the south, six in more northern areas. The comparative cross-variety approach provides a number of different lines of evidence which can then be used for testing similarities and differences across varieties. First, there is a dramatic difference between neg/aux contraction with be compared to the other auxiliaries that is consistent across all the communities. In every location be has aux contraction, and in each case it has higher rates of aux contraction than will or have. Second, all the Scots varieties have categorical aux contraction with be, just as would be expected from the historical record. However, there is a marked difference across the same varieties with will. Third, in the four locales where there is variation between neg and aux contraction the choice of form can be explained by the influence of the preceding phonological environment. In sum, neg/aux contraction is a poor diagnostic for distinguishing varieties of British English on broad geographic grounds. In contrast, at other levels of grammar (morphology and syntax) there are broad similarities across northern varieties. We conclude that the type of linguistic feature targeted for investigation plays a critical role in determining the similarities and differences amongst varieties.