Proto-Celtic phonological reconstruction

Consonants
The following consonants have been reconstructed for Proto-Celtic:

In contrast to the parent language, Proto-Celtic does not use aspiration as a feature for distinguishing phonemes. So the Proto-Indo-European voiced aspirated stops *, *, * merged with *, *, *. The voiced aspirate labiovelar * did not merge with *, though: plain * became * in Proto-Celtic, while aspirated * became *. Thus, PIE * 'woman' became Old Irish ben and Welsh benyw, but PIE * 'to kill, to wound' became Old Irish gonaid and Welsh gwanu.

Proto-Indo-European * was lost in Proto-Celtic, apparently going through the stages * (as in the table above) and * (perhaps attested by the toponym Hercynia if this is of Celtic origin) before being lost completely word-initially and between vowels. Adjacent to consonants, Proto-Celtic * underwent different changes: the clusters * and * became * and * respectively already in Proto-Celtic. PIE * became Old Irish s (lenited f-, exactly as for PIE *) and Brythonic f; while argues there was an intermediate stage * (in which * remained an independent phoneme until after Proto-Insular Celtic had diverged into Goidelic and Brythonic),  finds it more economical to believe that * remained unchanged in PC, that is, the change * to * did not happen when * preceded. (Similarly, Grimm's law did not apply to *p, t, k after *s in Germanic.)

In Gaulish and the Brythonic languages, a new * sound has arisen as a reflex of the Proto-Indo-European * phoneme. Consequently one finds Gaulish petuar[ios], Welsh pedwar "four", compared to Old Irish *cethair and Latin quattuor. Insofar as this new /p/ fills the space in the phoneme inventory which was lost by the disappearance of the equivalent stop in PIE, we may think of this as a chain shift.

The terms P-Celtic and Q-Celtic are useful when we wish to group the Celtic languages according to the way they handle this one phoneme. However a simple division into P- and Q-Celtic may be untenable, as it does not do justice to the evidence of the ancient Continental Celtic languages. The large number of unusual shared innovations among the Insular Celtic languages are often also presented as evidence against a P-Celtic vs Q-Celtic division, but they may instead reflect a common substratum influence from the pre-Celtic languages of Ireland, Scotland, Cornwall and Wales,, or simply continuing contact between the insular languages; in either case they would be irrelevant to Celtic language classification in the genetic sense.

Q-Celtic languages may also have /p/ in loan words, though in early borrowings from Welsh into Primitive Irish /kʷ/ was used by sound substitution due to a lack of a /p/ phoneme at the time: Gaelic póg "kiss" was a later borrowing (from the second word of the Latin phrase osculum pacis "kiss of peace") at a stage where p was borrowed directly as p, without substituting c.
 * Latin Patricius "Saint Patrick"' > Welsh > Primitive Irish > Old Irish Cothrige, later Padraig;
 * Latin presbyter "priest" > early form of word seen in Old Welsh premter primter > Primitive Irish > Old Irish cruimther.

Vowels
The Proto-Celtic vowel system is highly comparable to that reconstructed for Proto-Indo-European by Antoine Meillet. The following monophthongs have been reconstructed:
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The following diphthongs have also been reconstructed: