Initial consonant mutation is the phenomenon in which the first consonant of a word is changed according to a certain grammatical environment.
Mutation plays a crucial role in all Celtic languages. Because of these mutations, it becomes extremely difficult to use a Celtic language dictionary without knowledge of mutation patterns.
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Welsh has three mutation types, all affecting the initial consonant of a word:
| Unmutated Consonant | Soft Mutation | Nasal Mutation | Aspirate Mutation |
|---|---|---|---|
| p | b | mh | ph |
| t | d | nh | th |
| c | g | ngh | ch |
| b | f | m | |
| d | dd | n | |
| g | disappears | ng | |
| m | f | ||
| ll | l | ||
| rh | r |
Breton has four mutations: though it lacks the nasal mutation of Welsh, it also has a 'hard' mutation, in which voiced stops become voiceless, and a 'mixed' mutation, which is a mixture of hard and soft mutations. These mutations are shared by Cornish, which describes them in terms of 'states': the unmutated consonant is described as being in the 'first state'; a consonant affected by soft mutation in the 'second state'; by spirant mutation in the third; hard mutation in the fourth; and mixed mutation in the fifth.
| Unmutated Consonant | Soft Mutation | Aspirate Mutation | Hard Mutation | Mixed Mutation |
|---|---|---|---|---|
| p | b | f | ||
| t | d | th | ||
| k | g | h | ||
| b | v | p | f | |
| d | dh | t | t | |
| g1 | disappears | k | h | |
| g2 | w | k | hw | |
| gw | w | kw | hw | |
| m | v | f | ||
| ch | j |
1Before unrounded vowels, l, and r(providing it is followed by an unrounded vowel).
2Before rounded vowels, and r(providing it is followed by a rounded vowel).
Irish and Scottish Gaelic both exibit two mutations, known as 'lenition' and 'eclipsis'. Lenition transforms stops into fricatives, as well as affecting two fricatives (turning /s/ into /h/ and silencing /f/). Eclipsis, on the other hand, turns voiced stops into nasals, and voiceless stops into voiced ones, as well as voicing /f/. Unlike Welsh, Breton and Cornish, these languages show the mutated forms differently according to their source. For instance Cork is Corcaigh in Irish, but 'in Cork' is 'i gCorcaigh'. This differentiates a mutated from from an unmutated 'g'. As another example, 'bh', 'mh', and 'bhf' all sound identical, however, they all have different sources: the first is derived from /b/ through lenition; the second from /m/ through lenition; and the third from /f/ through eclipsis (although the former two may represent medial /v/).
The Sindarin language created by J. R. R. Tolkien has lenition patterns based on those of Welsh. A word becomes softened if it is the second part of a compound, anything after an article, or the subject of a verb and next to it. Thus, we get certh, rune, and i gerth, the rune.