The history of British language and its descendants, Welsh, Cornish and Breton, before the Norman Conquest is very imperfectly known. An attempt is made here to trace, from all available evidence, their development from the first to the twelfth century, and especially to analyse the chronology of their sound changes. Part I deals with the sources, such as Romano-British and post-Roman inscriptions; names in Classical authors; early Welsh, Cornish and Breton documents; the Latin loanwords in British and Irish; and many British place-names in English, which can only be adequately understood when fitted into such a chronological scheme. Part II sets out in detail the probable dates of the linguistic developments concerned.