Breton language variations exist depending on geographic location. Multiple linguistic traits occur across lower Brittany but, nonetheless, form a common linguistic continuum. The popular saying: "N'eo ket memes brezhoneg / It's not the same Breton" seems to indicate that vernaculars are different from one village to the other, but this is incorrect. All local vernaculars originate from the simple junction of multiple linguistic characteristics present elsewhere. Knowledge of the language's geographical variations was gained from the XIX century when data was collected on the local vernaculars and subsequently, thanks to the systematic mapping of the data in linguistic atlases (Pierre Le Roux, 1924 and Jean Le Dû 2001).
Geolinguistic research of these maps (Falc'hun 1952) deepened the historical knowledge of Breton (the economic impact and natural obstacles such as mountains and rivers, the role of Carhaix or the seaports in lower Brittany, for instance) and goes beyond the simplistic categorisation of dividing Breton into four dialects corresponding to the borders of the four bishoprics. Indeed, a central diagonal line going through Carhaix from south-Finistère up to Trégor, possibly to Goëlo, is distinguishable from the two other linguistic groups that are the Breton vernacular from the North-West (Léon and beyond, possibly up to the Trégor side in Finistère to the Monts d'Arrée, down to the Crozon peninsula and Cap-Sizun) as well as the Breton vernacular from the South-East (Vannes and beyond, sometimes including the Goëlo region, and possibly up to Quimper). The two Breton vernaculars further from the centre have maintained more archaic linguistic forms and are quite similar, whereas the central diagonal vernacular has more modern and unified linguistic characteristics.
