Scholarly handling of Celtic languages has been contentious owing to scarceness of primary source data. Some scholars (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) posit that the primary distinction is between Continental Celtic and Insular Celtic, arguing that the differences between the Goidelic and Brittonic languages arose after these split off from the Continental Celtic languages. Other scholars (such as Schmidt 1988) make the primary distinction between P-Celtic and Q-Celtic languages based on the replacement of initial Q by initial P in some words. Most of the Gallic and Brittonic languages are P-Celtic, while the Goidelic and Hispano-Celtic (or Celtiberian) languages are Q-Celtic. The P-Celtic languages (also called Gallo-Brittonic) are sometimes seen (for example by Koch 1992) as a central innovating area as opposed to the more conservative peripheral Q-Celtic languages. According to Ranko Matasović in the introduction to his 2009 Etymological Dictionary of Proto-Celtic: "Celtiberian ... is almost certainly an independent branch on the Celtic genealogical tree, one that became separated from the others very early."
The Breton language is Brittonic, not Gaulish, though there may be some input from the latter, having been introduced from Southwestern regions of Britain in the post-Roman era.
In the P/Q classification schema, the first language to split off from Proto-Celtic was Gaelic. It has characteristics that some scholars see as archaic, but others see as also being in the Brittonic languages (see Schmidt). In the Insular/Continental classification schema, the split of the former into Gaelic and Brittonic is seen as being late.
The distinction of Celtic into these four sub-families most likely occurred about 900 BC according to Gray & Atkinson but, because of estimation uncertainty, it could be any time between 1200 and 800 BC. However, they only considered Gaelic and Brythonic. A controversial paper by Forster & Toth included Gaulish and put the break-up much earlier at 3200 BC ± 1500 years. They support the Insular Celtic hypothesis. The early Celts were commonly associated with the archaeological Urnfield culture, the Hallstatt culture, and the La Tène culture, though the earlier assumption of association between language and culture is now considered to be less strong.
There are legitimate scholarly arguments for both the Insular Celtic hypothesis and the P-/Q-Celtic hypothesis. Proponents of each schema dispute the accuracy and usefulness of the other's categories. However, since the 1970s the division into Insular and Continental Celtic has become the more widely held view (Cowgill 1975; McCone 1991, 1992; Schrijver 1995), but in the middle of the 1980s, the P-/Q-Celtic theory found new supporters (Lambert 1994), because of the inscription on the Larzac piece of lead (1983), the analysis of which reveals another common phonetical innovation -nm- > -nu (Gaelic ainm / Gaulish anuana, Old Welsh enuein 'names'), that is less accidental than only one. The discovery of a third common innovation would allow the specialists to come to the conclusion of a Gallo-Brittonic dialect (Schmidt 1986; Fleuriot 1986).
The interpretation of this and further evidence is still quite contested, and the main argument for Insular Celtic is connected with the development of verbal morphology and the syntax in Irish and British Celtic, which Schumacher regards as convincing, while he considers the P-Celtic/Q-Celtic division unimportant and treats Gallo-Brittonic as an outdated theory. Stifter affirms that the Gallo-Brittonic view is "out of favour" in the scholarly community as of 2008 and the Insular Celtic hypothesis "widely accepted".
When referring only to the modern Celtic languages, since no Continental Celtic language has living descendants, "Q-Celtic" is equivalent to "Goidelic" and "P-Celtic" is equivalent to "Brittonic".
How the family tree of the Celtic languages is ordered depends on which hypothesis is used:
Eska considers a division of Transalpine–Goidelic–Brittonic into Transalpine and Insular Celtic to be most probable because of the greater number of innovations in Insular Celtic than in P-Celtic, and because the Insular Celtic languages were probably not in great enough contact for those innovations to spread as part of a sprachbund. However, if they have another explanation (such as an SOV substratum language), then it is possible that P-Celtic is a valid clade, and the top branching would be:
Although there are many differences between the individual Celtic languages, they do show many family resemblances.
† Borrowings from Latin.
Several poorly-documented languages may have been Celtic.
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