All surviving Celtic languages are in the Insular group, including Breton, which is spoken on continental Europe in Brittany, France.
They add the identical sound shift (/kʷ/ to /p/) could have occurred independently in the predecessors of Gaulish and Brittonic, or have spread through language contact between those two groups.
A significant difference between Goidelic and Brittonic languages is the transformation of *an, *am to a denasalised vowel with lengthening, é, before an originally voiceless stop or fricative, cf.
Old Irish éc "death", écath "fish hook", dét "tooth", cét "hundred" vs. Welsh angau, angad, dant, and cant.
[3] These are: The Insular Celtic verb shows a peculiar feature unknown in any other attested Indo-European language: verbs have different conjugational forms depending on whether they appear in absolute initial position in the sentence (Insular Celtic having verb–subject–object or VSO word order) or whether they are preceded by a preverbal particle.
The situation is most robustly attested in Old Irish, but it has remained to some extent in Scottish Gaelic and traces of it are present in Middle Welsh as well.
The paradigm of the present active indicative of the Old Irish verb beirid "carry" is as follows; the conjunct forms are illustrated with the particle ní "not".
The example given in the first column below is the independent or absolute form, which must be used when the verb is in clause-initial position (or preceded in the clause by certain preverbal particles).
), found the correct solution to the origin of the absolute/conjunct distinction: an enclitic particle, reconstructed as *es after consonants and *s after vowels, came in second position in the sentence.
Cowgill suggests it might be a semantically degraded form of *esti "is", while Schrijver (1994) has argued it is derived from the particle *eti "and then", which is attested in Gaulish.
[6] The theory has been supported by several linguists since: Henry Jenner (1904);[7] Julius Pokorny (1927);[8] Heinrich Wagner (1959);[9] Orin Gensler (1993);[10] Theo Vennemann (1995);[11] and Ariel Shisha-Halevy (2003).