Consonant Mutations

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: p 'palace', t 'bull', c, k 'cat', b 'bard', d 'fort', g 'goat', m 'son', gw/f 'man', s '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 m 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 f and s, though these mutations share a history with Brythonic.  IGM. f and WCB. gw are both reflexes of PC. w-, hence IG. fear, M. fer, W. gwr, CB. gour < PC *wiros 'man'.  British s 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 f and s are lenited in the colloquial language to v and z respectively.

With 'his' (< PC. *esjo):

In addition to the above mutations, Cornish has ch → j and, in some versions, qw or qu → gw, whilst Manx has qu → wh, çh → 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):

Spirantization

Spirantization is the change of a voiceless stop → voiceless fricative.  This change was originally caused by a word-final -s, but in Breton and Cornish word final -n also caused spirantization (compare W. fy nghath 'my cat' with B. va c'hazh and C. ow hath). 

With 'her' (< PC. *esjās):

Hard Mutation

With B. ho 'your (pl)' & C. mar 'if'.

Comparison of Pronouns