These languages have been spoken in the British Isles since at least the Iron Age until today, originally as the majority languages but now as minority ones in Wales and Cornwall. In Ireland, the Isle of Man and Scotland the Brythonic languages have been replaced by Goedelic ones. By emigration there are also communities of Brythonic language speakers in Brittany, and Patagonia.
Other major characteristics include:
The major Brythonic languages today are Welsh and Breton, both of which survive as community languages. The Cornish language died out at the end of the eighteenth century, but attempts at reviving it started in the 20th century and are ongoing. Also notable are the extinct language Cumbric, and possibly the extinct Pictish although this may be best considered to be a sister of the Brythonic languages. The late Kenneth H. Jackson argued during the 1950s, from some of the few remaining examples of stone inscriptions, that the Picts may have also used a non-Indo-European language, but some modern scholars of Pictish do not agree.
The modern Brythonic languages are generally considered to all derive from a common ancestral language termed British, Common Brythonic, Old Brythonic or Proto-Brythonic, which is thought to have developed from Proto-Celtic and which was possibly introduced to Great Britain by the 6th century BC. The retired German linguist Vennemann has suggested that an earlier hypothetic language, Atlantic, may have influenced the Brythonic languages, but this is not generally accepted. Mario Alinei denies the existence of a pre-Celtic language and says that Celtic languages arrived in the Paleolithic but this is not generally accepted either. It is possible that a Germanic language may have been present in Eastern England because of cultural links across what is now the southern North Sea but was dry land in the Mesolithic, and this could have influenced the Brythonic languages. Stephen Oppenheimer has suggested that the Belgic invasions in the first century BC could have brought a Germanic language to Britain. However, it is clear from classical authors that Celtic was used for place and river names by 300 BC.
Brythonic languages were probably spoken prior to the Roman invasion at least in the majority of Great Britain south of the rivers Forth and Clyde, though the Isle of Man later had a Goidelic language, Manx. Northern Scotland mainly spoke Pritennic, which became Pictish, that may have been a Brythonic language. The theory has been advanced (notably by R. F. O'Rahilly) that Ireland was populated by speakers of Brythonic before being displaced by speakers of a Q-Celtic language (possibly from the Quarietii tribe of southern France), although the authors Dillon and Chadwick reject this theory as being implausible.
During the period of the Roman occupation of southern Great Britain (AD 43 to c. 410), Common Brythonic borrowed a large stock of Latin words, both for concepts unfamiliar in the pre-urban society of Celtic Great Britain, such as urbanisation and tactics of warfare, and for rather more mundane words which displaced native terms (most notably, the word for "fish" in all the Brythonic languages derives from the Latin piscis rather than the native *ēskos > Wysg river). Approximately eight hundred of these Latin loan-words have survived in the three modern Brythonic languages. Romano-British is the name for the Latinised form of the language used by Roman authors.
It is probable that at the start of the Post-Roman period Common Brythonic was differentiated into at least two major dialect groups - Southwestern and Western (in addition we may posit additional dialects, such as Eastern Brythonic, spoken in what is now eastern England, which have left little or no evidence). Between the end of the Roman occupation and the mid sixth century the two dialects began to diverge into recognisably separate languages, the Western into Cumbric and Welsh, and the Southwestern into Cornish and its closely related sister language Breton, which was carried from the south west of Great Britain to continental Armorica. Jackson showed that a few of the dialect distinctions between West and Southwest Brythonic go back a long way. New divergencies began around AD 500 but other changes which were shared occurred in the 6th century. Other common changes occurred in the seventh century onward and are possibly due to inherent tendencies. Thus the concept of a common Brythonic language ends by AD 600. It is thought that substantial numbers of Britons remained in the expanding area controlled by Anglo-Saxons, but the only information on their language may be obtained from place names. Over time it is thought they gradually adopted the English language.
The Brythonic languages spoken in Scotland, the Isle of Man and England began to be displaced in the 5th century through the influence of Irish (Scots), Norse and Germanic invaders. The displacement of the languages of Brythonic descent was probably complete in all of this territory, (except Cornwall and the English counties bordering Wales), by the 11th century (date of extinction in various parts of the territory is debated). Ivernic is a Brythonic language that may have been spoken in Ireland.
The number of Celtic river names in England generally increases from east to west, a map showing these being given by Jackson. These names include ones such as Avon, Chew, Frome, Axe, Brue and Exe.
Some researchers (Filppula et al., 2001) argue that English syntax reflects more extensive Brythonic influences. For instance, in English tag questions, the form of the tag depends on the verb form in the main statement (aren't I?, isn't he?, won't we? etc). The German nicht wahr? and the French n'est-ce pas?, by contrast, are fixed forms which can be used with almost any main statement. It has been claimed that the English system has been borrowed from Brythonic, since Welsh tag questions vary in almost exactly the same way. This view is far from being generally accepted, though.