Consonant mutations are a fundamental part of all modern Celtic languages.  They developed out of phonological changes which began occurring in the Common Brythonic and Primitive Irish periods, but developed into grammatically triggered mutations when much of the inflexional information was lost. 

The mutations are compared below.  The nouns used are: ‘palace’, ‘bull’, c, k ‘cat’, ‘bard’, ‘fort’, ‘goat’, ‘son’, gw/f ‘man’, ‘room’.

Lenition or the Soft Mutation

Lenition is the only mutation which all the modern Celtic languages share, though it causes different changes throughout.  It was originally caused where certain consonants occurred between two vowels.

In the Brythonic languages, lenition causes the changes voiceless stop → voiced stop and voiced stop & m → voiced fricative.  The lenited form of was originally a nasal fricative (sometimes represented with μ), but nasality was lost in Welsh and Cornish in the Middle Ages.  In Breton nasality remains internally, even where the original v has been lost (e.g. gwanañ ‘weakest’ < *wanisamos: W. gwanaf).  Breton is also the only Brythonic language to retain a lenited form of g-; the sound would originally have been /ɣ/ but this was lost in Welsh and Cornish, with Breton assimilating the sound to the voiceless c’h /x/.

In the Gaelic languages the change is stop → fricative, and fricative → h or Ø.  During the Middle Ages the sound of th /θ/ became /h/ and dh /ð/ assimilated to /ɣ/.  Unlike the Brythonic languages, Gaelic lenition also affects the radical fricatives and s, though these mutations share a history with Brythonic.  IGM. and WCB. gw are both reflexes of PC. w-, hence IG. fear, M. fer, W. gwr, CB. gour < PC *wiros ‘man’.  British also lenited to h, but an an early date and instead of becoming a grammatically triggered mutation the sound was generalised to most words beginning with s.  This explains the difference between WCB. hen and IG. sean < PC. *senos ‘old’, WCB. hir and G. sìor < PC. *sīros ‘long’; but also why some Brythonic words still have initial s- (e.g. W. saith, B. seizh < PC. *sektam ‘seven’: G. seachd).  In some forms of modern Cornish, initial and are lenited in the colloquial language to and respectively.

With ‘his’ (< PC. *esjo):

pei balasy balyse baleza phálása phàlase phlaase
tei darwy darowe darva tharbha tharbhe harroo
c, kei gathy gathe gazha chata chate chayt
bei farddy vardhe varzha bharda bhàrde vard
dei ddiny dhine zin a dhúna dhùne ghoon
gei afry avere c’havra ghabhara ghobhare ghoayr
mei faby vabe vaba mhaca mhace vac
gw/fei ŵry woure oura fheara fheare er
s   a sheomraa sheòmare hamyr

In addition to the above mutations, Cornish has ch → j and, in some versions, qw or qu → gw, whilst Manx has qu → whçh → h and j → y

Eclipsis or Nasalization

The word eclipsis, sometimes used in the Gaelic languages, refers to the fact that a one sound replaces or ‘eclipses’ another.  In Welsh this is usually called the nasal mutation, which is more apt since the mutation was originally caused by a word-final -n.  The changes are Welsh stop → nasal and Gaelic voiceless stop → voiced stop, voiced stop → nasal and /f/ → /v/.  In Irish the radical letter remains and the mutation is written before it, so cat /kat/ becomes gcat /gat/. 

With ‘my’ (Welsh < Br. *men) and ‘their’ (Irish and Manx < PC. *esjon):

pfy mhalasa bpálásnyn blaase   
tfy nharwa dtarbhnyn darroo   
c, kfy nghatha gcatnyn gayt   
bfy mardda mbardnyn mard   
dfy nina ndúnnyn n’ghoon   
gfy ngafra ngabharnyn noayr   
gw/ffy ngŵra bhfearnyn ver   

Hard Mutation

With B. ho ‘your (pl)’ & C. mar ‘if’.

bho parzhmar prathav‘if I bite’   
dho tinmar toutya‘if I doubt’   
gho kavrmar kobrav‘if I reward’   

Comparison of Pronouns

myfy nghath
fy ngafr
ow hath
ow gaver
va c’hazh
va gavr
mo chat
mo ghabhar
mo chat
mo ghobhar
my chayt
my ghoayr
yourdy gath
dy afr
dha gath
dha aver
da gazh
da c’havr
do chat
do ghabhar
do chat
do ghobhar
dty chayt
dty ghoayr
hisei gath
ei afr
y gath
y aver
e gazh
e c’havr
a chat
a ghabhar
a chat
a ghobhar
e chayt
e ghoayr
herei chath
ei gafr
hy hath
hy gaver
he c’hazh
he gavr
a cat
a gabhar
a cat
a gobhar
e cayt
e goayr
ourein cath
ein gafr
agan kath
agan gaver
hor c’hazh
hor gavr
ár gcat
ár ngabhar
ar cat
ar gobhar
nyn gayt
nyn n’ghoayr
youreich cath
eich gafr
agas kath
agas gaver
ho kazh
ho kavr
bhur gcat
bhur ngabhar
ur cat
ur gobhar
nyn gayt
nyn n’ghoayr
theireu cath
eu gafr
aga hath
aga gaver
o c’hazh
o gavr
a gcat
a ngabhar
an cat
an gobhar
nyn gayt
nyn n’ghoayr