An investigation by G. P. Cubbin into the local placename sources of Lancashire of a time when the vernacular had a low status isolated the Whalley Coucher Book as the one that most seemed to deserve further scrutiny. That book therefore forms the basis of the present study. The Coucher Boook is a mediaeval work of monastic provenance and is a compilation of deeds received by Whalley Abbey over the period. The interest of the source lies in its representation of many place-names by writers who may be supposed to have been familiar with them. Whalley's placename corpus affords scope for examination of variation that is of dialectal significance. A searching analysis is undertaken of the evidence that the Whalley Coucher Book offers. Questions of dating, of location of place-names, of the elements that compose them, and of the status of the text have to be examined with a view to elucidating the significance for phonology of this evidence. Such examination is carried out at length, and it is hoped that these aspects of the present work may be found to have application in linguistic and historical inquiry both for the actual results relative to the Whalley Coucher Book and for the methodological demonstration. A considerable amount of dialectal phonological information from the source is presented in this thesis. It is critically examined and collated and the attempt is made to derive actual usage in the territory and period concerned. On the whole the conclusion is that most of the evidence does reflect the dialect and that it produces a believable distribution of forms. Some of the dialectal information thus acquired appears as new. More commonly, however, this study confirms the existing picture or makes it somewhat more precise. The evidence does not escape the uneven coverage that is to be expected in place-name evidence for dialect. Although the amount of the evidence of the Whalley Coucher Book and its general consistency are comparatively good, the finding of this work is that they are not enough to establish the original suggestion that the Coucher Book might deserve reliance without reference to, and even in total defiance of, other local sources. The present study concludes that the best evidence consists of a select group of sources amongst which Whalley may be taken as pre-eminent.