V = vowel
C = consonant
| PIE | Proto-Celtic | Common Brythonic | Modern Iniseg | Modern Welsh | Modern Breton | Modern Cornish |
| /b-/, /-bb-/ | /b/, /-b-/ | [b] /b/ | [b] /b/ | [b] /b/ | [b] /b/ | |
| /-VbV-/ | /-VvV-/ | [v] /v/ | [f] /v/ | [v] /v/ | [v] /v/ | |
| /d-/, /-dd-/ | /d-/, /-d-/ | [d] /d/ | [d] /d/ | [d] /d/ | [d] /d/ | |
| /-VdV-/ | /-VðV-/ | [dh] /ð/ | [dd] /ð/ | [z] /z/ | [dh] /ð/ | |
| /g-/, /-gg-/ | /g-/, /-g-/ | [g] /g/ | [g] /g/ | [g] /g/ | [g] /g/ | |
| /-VgV-/ | /-VɣV-/ -> /-VjV-/ | (lost) | (lost) | (lost) | (lost) | |
| /ɸ-/, /-ɸ-/ | (lost) | (lost) | (lost) | (lost) | (lost) | |
| /-xt-/ | /-xθ-/ -> /-(i)θ/ | [th] /θ/ | [th] /θ/ | [zh] /z/ or /h/ | [th] /θ/ | |
| /j-/ | /i-/ | [y] /j/ | [i] | [i] | [i] | |
| /-j/ | /-ð/ | [dh] /ð/ | [dd] /ð/ | [z] /z/ | [dh] /ð/ | |
| /k-/ | /k-/ | [k] /k/ | [c] /k/ | [k] /k/ | [k] /k/ | |
| /-kk-/ | /-x-/ | (lost) | [ch] /χ/ | [c'h] /x/ or /h/ | [gh] /h/ | |
| /kʷ-/ | /p-/ | [p] /p/ | [p] /p/ | [p] /p/ | [p] /p/ | |
| /-kʷ-/ | /-b-/ | [b] /b/ | [b] /b/ | [b] /b/ | [b] /b/ | |
| /l-/ | /l-/ | [lh] /l̥/ | [ll] /ɬ/ | [l] /l/ | [l] /l/ | |
| /-ll-/, /-VlV-/ | /-l-/ | [l] /l/ | [l] /l/ | [l] /l/ | [l] /l/ | |
| /m-/, /-Cm-/ | /m-/, /-m-/ | [m] /m/ | [m] /m/ | [m] /m/ | [m] /m/ | |
| /-mb-/ | /-mm-/ | [mm] /mː/ | [m] /m/ | [m] /m/ | [m] /m/ | |
| /-m-/ | /-β̃-/ | [v] /v/ | [f] /v/ | [ñv] /̃v/ | [v] /v/ | |
| /n-/, /-n-/ | /n-/, /-n-/ | [n] /n/ | [n] /n/ | [n] /n/ | [n] /n/ | |
| /-nd-/ | /-nn-/ | [nn] /nː/ | [n] [nn] /n/ | [n] [nn] /n/ | [n] [nn] /n/ | |
| /-nt-/ | /-nt-/ | [nd] /nd/ -> [nn] /nː/ | [nt] /nt/, [nh] /n̥/ | [nt] /nt/ | [nt] /nt/ | |
| /-pp-/ | /-ɸ-/ -> /-f-/ | [f] /f/ | [ff] /f/ | [f] /f/ | [f] /f/ | |
| /r-/ | /r-/ | [rh] /r̥/ -> [r] /r/ | [rh] /r̥/ | [r] /r/ | [r] /r/ | |
| /-r-/ | /-r-/ | [r] /r/ | [r] /r/ | [r] /r/ | [r] /r/ | |
| /s-/ | /h-/, /s/ | [h] /h/, [s] /s/ | [h] /h/, [s] /s/ | [h] /h/, [s] /s/ | [h] /h/, [s] /s/ | |
| /-s-/ | /-s-/ | [s] /s/ | [s] /s/ | [s] /s/ | [s] /z/ | |
| /sl-/ | /l-/ | [lh] /l̥/ | [ll] /ɬ/ | [l] /l/ | [l] /l/ | |
| /sm-/ | /m-/ | [m] /m/ | [m] /m/ | [m] /m/ | [m] /m/ | |
| /sn-/ | /n-/ | [n] /n/ | [n] /n/ | [n] /n/ | [n] /n/ | |
| /sɸ-/ | /f/ | [f] /f/ | [ff] /f/ | [f] /f/ | [f] /f/ | |
| /sr-/ | /fr-/ | [fr] /fr/ | [ffr] /fr/ | [fr] /fr/ | [fr] /fr/ | |
| /sw-/ | /hw-/ | [hw] /ʍ/ | [chw] /χw/ | [c'ho] /xw/ | [hw] /ʍ/ | |
| /t/ | /t/ | [t] /t/ | [t] /t/ | [t] /t/ | [t] /t/ | |
| /-t-/ | /-d-/ | [d] /d/ | [d] /d/ | [d] /d/ | [d] /d/ | |
| /-tt-/ | /-θ-/ | [th] /θ/ | [th] /θ/ | [zh] /z/ or /h/ | [th] /θ/ | |
| /w-/ | /ˠw-/ -> /ɣw-/ -> /gw-/ | [gw] /gw/ | [gw] /gw/ | [gw] /gw/ | [gw] /gw/ | |
| /-VwV-/ | /-w-/ | [w] /w/ | [w] /w/ | [w] /w/ | [w] /w/ | |
| /-V/ | /-Vh/ | [Vh] /Vh/ -> [V] /V/ | [Vch] /Vχ/ | [Vc'h] /Vx/ or /Vh/ | [Vgh] /Vh/ |
Vowels
| PIE | Proto-Celtic | Common Brythonic | Iniseg | Welsh | Breton | Cornish |
| /a/ | /a/ | /a/ | [a] | [a] | [a] | [a] |
| /e/ | /e/ | /e/ | [e] | [e] | [e] | [e] |
| /i/ | /i/ | /i/, /e/ | [i] | [y], [e] before final -ā, [y] /ə/ pretonic | [i] -> [e] | [e], [y] before nn and nasal + stop |
| /o/ | /o/ | /o/ | [o] | [o] | [o] | [o] |
| /u/ | /u/ | /u/ | [u] | [w] (mono-/final syllable), umlaut -> [y] /ə/ | [o], [ou], umlaut -> [e] | [o], [ou], [ow], umlaut -> [e] |
Vowels + consonants
| PIE | Proto-Celtic | Common Brythonic | Iniseg | Welsh | Breton | Cornish |
| /énkʷ-/ (e + nasal + explosive) | /enkʷ-/ | /ɨmp/ | [ump] | [emp] | [ymp] | |
| /éh₁n̥t-/ (e + nasal + explosive) | /int-/ | /ɨnt/ | [ynt] | [ent] | [yns] | |
| /esw-/ (e before sw, sp) | [ych] | |||||
| /pékʷ-/ | /kʷokʷ-/ | /pob-/ | [pob] | [pob] | [pob] | |
| /ork-/ | /ork-/ | /urx/ | [wrch] /ʊrχ/, [yrch] /ərχ/) | [ourc'h] | [orgh] | |
| /org-/ | /orɣ/ | [wrw] | [erc'h] | [erch] | ||
| /ong-/ | /ung/ | [wng] /ʊŋ/, [yng] /əŋ/ | [ou] | |||
| /on-/ | /on-/ | /un-/ | [wn] /ʊn/ | |||
| /upo-/ | /uɸo-/, /wo-/ | /gwo-/, /gwa-/ | [gwo-], [go-] | [gwa-], [go-] | [gwa-], [gou-] | [gwa-], [go-] |