The study of Proto-Brittonic or British, Proto-Celtic and Proto-Indo-European is an area of study that allows for considerable speculation and in most cases only tangentially relevant, but the study of Brittonic specifically is very much central to it, and so too the work of Kenneth Hurlstone Jackson (1909–1991), the first person to develop a timeline for changes in the phonology of Brittonic. Jackson’s Language and History in Early Britain (LHEB) (1953) references several other significant works on Celtic grammar, including:
- Pederson’s Vergleichende Grammatik der Keltischen Sprachen (VKG),
- Pedersen and Lewis’ A Concise Comparative Celtic Grammar (CCCG),
- Thurneysen’s A Grammar of Old Irish,
- M. Jones’ A Welsh Grammar, and
- Baudis’ Grammar of Early Welsh.
LHEB comprehensively surveyed the phonology of Early Medieval Brittonic and mapped the evolution from ancient to modern Brittonic languages, has an extensive list of references and draws on a wide range of sources. It was the first attempt to set out a timeline for changes to Brittonic, and one which broadly still stands. No one has come close to its comprehensiveness or thoroughness. It is an essential reference within this project as it deals with the transformation of Late South-western and Late Western British into Primitive and then Old Cornish and Welsh. The review by Myres (1955) still stands and Jackson’s work is essential to this study.