Common Brittonic phonology

Vowels
The late Common Brittonic vowel inventory was still very similar to that of Proto-Celtic, with the short vowels seeing little change. The long vowels meanwhile had seen some development: earlier /uː/ having merged with /iː/, /aː/ becoming /ɔː/, and two new long vowels developed from earlier diphthongs: /ʉː/ (from /au/, /ou/, /oi/) and /ɛː/ (from /ai/). Similarly, the earlier diphthong /ei/ merged with Brittonic /eː/.