v. (1) o and a interchange after u̯ § 34 iv. So we have gwa- beside go- for gwo- < *u̯o‑: Gaul. vo- < Ar. *upo; thus gwa-red-wr ‘saviour’ < *u̯o-reto-u̯ir‑: Gaul. Voretovir‑;—W. gwas ‘servant’: Ir. foss < *upo‑st- § 96 ii (2). The 15th cent. pedwor § 34 iv (so Salesbury’s Dic. s. v.) has a new, perhaps local, o for a § 63 vii (4).
We also find the interchange after ü (cons. or voc.), as breuan for *breuon § 76 iv (2); bū́an for *büon § 76 ix (2); (Anglesey dial. neuoẟ for neuaẟ).
(2) After m- there is an older change of a to o, as in W. môr ‘sea’, Gaul. Aremorici, Ir. muir: Lat. mare; W. myned < *monet‑, Bret. monet < *mami̯et- § 100 iv;—W. morwyn < *marein- § 125 v (1).
(3) e after u̯ becomes o/a in the following cases: Ar. *uper > Pr. Kelt. *u̯er > Gaul. ver‑, Bret. war ‘on’, W. ar, gwar‑, gwor‑, gor- § 36 iii;—W. Cadwallon < Brit. Catu-vellaunos;—W. gosper < Lat. vesper‑. Probably the above show the influence of Brit. u̯o‑; cf. Ir. for- < *u̯er- on the analogy of fo- < *u̯o‑. Generally u̯e remains, as in chwech ‘six’ < *su̯ek̑s.
vi. (1) After i̯ post-tonic a became e; thus wyneb ‘face’ < *éni̯-equ̯‑, § 100 v, < *éni̯-aqu̯- < *éni̯-əqu̯‑, √ōqu̯ = Skr. ánīkam ‘face’ < *eni-əqu̯om. But when pre-tonic the a remained, as in wynab‑, in composition, from *eni̯aqu̯‑; gwyẟi̯ad < *u̯idi̯ətó § 180 iv (1).
(2) Pre-tonic i̯o prob. became i̯a; thus we have aea < *‑ii̯a‑´, but no *aeo < *‑iio‑´, so that the latter perhaps became *‑ii̯a‑´ § 75 vi (2). So the rel. a <*i̯a < Ar. i̯os, § 162 vi (1).
§ 66. i. Pr. Kelt. i and u remained in Brit. Brit. i was open, and is transcribed ε by the Greeks, as in Πρετ(τ)ανικὴ (νῆσος) : W. (ynys) Prydain, but i by the Romans as in Britannia (Gk. ι was close, Lat. i open). Brit. i gave W. y, which is ɥ in the ult. and accented monosyllables, ỿ in non-ultimate syllables and proclitics. Brit. u remains, now written w, in the ultima and monosyllables, and becomes y (≡ ỿ) in all other syllables. See § 40 iii. Examples: W. drɥch ‘appearance’, edrỿchaf ‘I look’ < Pr. Kelt. *dr̥k‑, § 61 i;—W. cŵn ‘dogs’, cỿnos ‘little dogs’ < Pr. Kelt. *kun‑;—W. cỿbɥdd ‘miser’ < Lat. cupidus;—W. terfɥn ‘end’ < Lat. terminus.
ɥ and ỿ may interchange with e, and ỿ with a, § 16 iv.
u before a labial may develop irregularly, § 73 ii.
ii. (1) ỿ in the penult, whether from i or u becomes w in Mn. W. before w in the ult., as in cwmwl ‘cloud’ for cymwl < *cumbul- < Lat. cumulus; swmbwl < *stimbul- < Lat. stimulus; cwmwd ‘comot’ < Ml. W. kymwt; dwthwn < dythwn < dydd hun § 164 iii. When a syllable is added, both w’s become ỿ, as cỿmỿlau ‘clouds’.
