Wiktionary:About Proto-Brythonic

Proto-Brythonic is the last common ancestral stage of the Brythonic languages: Western Brythonic Welsh, Southwestern Brythonic Breton and Cornish, and likely Cumbric as well.

Consonants

 * Sonorants: *l, *ll, *r, *rr, *m, *n
 * Approximants: *j, *w
 * Voiceless plosives: *p, *t, *k
 * Voiced plosives: *b, *d, *g
 * Labialized voiced plosives: *gw
 * Voiceless fricatives: *f, *θ, *x, *h, *s
 * Labialized voiceless fricatives: *hw
 * Voiced fricatives: *β, *ð, *ɣ, *β̃

Vowels

 * Back rounded: *u, *o, *ọ
 * Central rounded: *ʉ
 * Front rounded: *ü, *ö
 * Central unrounded: *a, *ɨ
 * Front unrounded: *e, *ė, *i

/oɨ/ and /uɨ/ were the regular development of long /ɛː/ and /eː/, respectively, while also resulting from vocalization of certain consonants following /o/ and /u/, whence also developed /aɨ/ and (partially) /ei/.

Diphthongs
I-dipthongs:


 * Front unrounded: *ėi, *ei
 * Front rounded: *üi, *öi
 * Central unrounded: *aɨ
 * Central rounded: *ʉɨ
 * Back rounded: *uɨ, *oɨ, *ọɨ

U-diphthongs (which we spell with w):


 * Front unrounded: *iw, *ɨw,*ėw, *ew
 * Front rounded: *üw, *öw
 * Central unrounded: *aw
 * Central rounded: *ʉw
 * Back rounded: *ow, *ọw

Developments from Proto-Celtic
Vowel development from Proto-Celtic to Late Brythonic:

In closed pretonic and pre-pretonic syllables, short -u- and -i- had been been reduced to rounded and unrounded schwas, respectively.

A-affection
When followed by the vowel /a/ in the final syllable, the short high vowels /i/ and /u/ are lowered to /e/ and /o/, respectively.


 * Proto-Celtic > Proto-Brythonic

This leads to a masculine-feminine distinction in some o-stem adjectives, as the masculine form continues /i/, and the feminine form now has /e/.
 * Masculine vs feminine

Final i-affection
When short back or non-high front vowels in penultimate position are followed by front high vowels or semivowels, mainly /iː/ or /j/, they are subsequently fronted and (if possible) raised:

Short /e/ is also the only vowel that is affected by short /i/: Proto-Celtic > Proto-Brythonic
 * /e/ becomes /i/:
 * /a/ becomes raised /e̝/: Proto-Celtic > Proto-Brythonic
 * /o/ and /u/ become /y/: Proto-Celtic > Proto-Brythonic

Internal i-affection
When short non-front-high vowels in any position are followed by front high vowels or semivowel /i/, /e̝/, /y/ or /j/, they are raised and/or fronted:
 * /e/ and /a/ become /e̝/:
 * /u/ becomes /y/:
 * /o/ becomes /ø/:

Miscellaneous raisings
/e/ and /o/ raised to /i/ and /u/ respectively before -rn-. o also raised to u before -rg-. /e/ also raised to /i/ before intervocalic -g-. These raisings may be reversed if a-affection is present.

Consonant mutation
Initial consonant mutation is essentially the same phenomenon as lenition, as well as other forms of consonant development, only now applied across word-boundaries. *esyo tegos > *esyo degoh (lenited); *esyās tegos > *esyāh tegoh > *esyāh θegoh (spirantized).

In the table below, the symbol ◌̞ is used to indicate a more weakly articulated version of a consonant, and ◌̬ to indicate a partially voiced version of a voiceless consonant; in both cases which the lenited consonant is derived from this version.

Basic vowel reflexes
Notes:
 * 1 In non-stressed Old Welsh syllables.
 * 2 Accent shift from ultima to penultima beyond Old Welsh caused monophthongization of ⟨aw⟩ to ⟨o⟩.
 * 3 ⟨ei⟩ becomes ⟨ai⟩ in Modern Welsh stressed monosyllables and final syllables.

Miscellaneous changes
The sequence *wo- led to *wa- in leniting positions, but was retained in non-leniting positions. This allomorphy was resolved separately in the daughter languages, with one reflex being generalized over the other.

Nouns
The plural is usually formed based on the Proto-Celtic nominative plural.

Notes:
 * 1 The plural ā-stems and plural neuter o-stems gave the same result as their respective singulars after apocope, leading to the spread of plural markers from other stem classes.
 * 2 From an earlier British plural *-iyes, which arose from analogy with the neuter i-stem plural *-iyā to parallel the plurals of the u-stem *-owes (m./f.) and *-owā (n.).
 * 3 Through the sequence of intervocalic s > h > Ø > epenthetic j > ð.
 * 4 The usual plural cerynt/ceraint was secondarily supplied with the o-stem ending *-oi > -ī, causing final i-affection.

Adjectives
(put information about feminines, plurals and comparatives here)