Editing Irish initial mutations (section)

==Historical development==
===Lenition===
Lenition as an initial mutation originally stems from the historical allophonic [[lenition]] of an intervocalic consonant, both word internally and across word boundaries, i.e if a word ended in a vowel and the next word began with a consonant + a vowel, the consonant lenited. 

Today, these former final vowels are usually elided, but the lenition of following consonants remains and has been [[grammaticisation|grammaticised]]. For example, [[Proto-Celtic]] ''*esyo'' "his" caused the lenition of a following consonant due to its final vowel and its modern form {{lang|ga|a}} now causes lenition, keeping it distinct from {{lang|ga|a}} "her" and {{lang|ga|a}} "their", which cause h-prothesis and eclipsis respectively.

Lenition caused stops and ''*m'' to become fricatives, ''*s'' to [[debuccalisation|debuccalise]] to {{IPA|[h]}}, ''*f'' to [[elide]], and the liquids ''*l'', ''*n'', ''*r'' to split into [[fortis and lenis]] variants. Though by the end of the [[Middle Irish]] period lenited ''*m'' largely lost its nasal quality, lenited ''*t'' debuccalised to {{IPA|[h]}}, and lenited ''*d'' lost its coronal articulation.

Lenition did not only occur word initially, though non-initial lenition was never grammaticised. For example Proto-Celtic ''*knāmis'' → {{lang|sga|cnáim}} → {{lang|ga|cnáimh}} "bone", and ''*abalnā'' → {{lang|sga|aball}} → {{lang|ga|abhaill}} "apple tree".

====Prothetic {{vr|t}}- and {{vr|h}}-====
While it is not initially apparent, the [[prothesis (linguistics)|prothesis]] of {{vr|t}} and {{vr|h}} stems from historical lenition combined with vowel reduction.

The prosthetic {{vr|t}}- of vowel initial words is a fossilised fragment of the Proto-Celtic masculine definite article ''*sindos''. Before vowels, the ''*s'' of the ending ''*-os'' was lenited to {{IPA|[h]}}, which (combined with the loss of the ''*-o-'') devoiced the preceding ''*-d-'' to ''*-t''.
* i.e. ''*sindos'' {{IPA|[sindoh]}} → {{lang|sga|int}} → {{lang|ga|an t-}}).

The prosthetic {{vr|t}} of {{vr|s}} initial words is a fossilised fragment of the d of Proto-Celtic nominative feminine definite article ''*sindā'' and masculine genitive definite article ''*sindī''. Since they ended in vowels, a following word initial ''*s'' was lenited to {{IPA|[h]}} which (combined with the loss of the ''*-ā'', ''*-ī'') devoiced the preceding ''*-d'' to ''*-t''.
* i.e. ''*sindā sūli'' [sindaː huːli] → {{lang|sga|int ṡúil}} → {{lang|ga|an tsúil}})

The prothetic {{vr|h}} of vowel initial words has two origins, the first being [[epenthetic]] to avoid vowel hiatus, and the second being the fossilised remnant of a historic consonant. For example, the ''*s'' of Proto-Celtic ''*esyās'' "her" was lenited between vowels to {{IPA|[h]}}. Overtime ''*esyās'' was reduced to {{lang|ga|a}} but the {{IPA|[h]}} remains when it is followed by a vowel initial word but is now written as part of the following word.

===Eclipsis===
Eclipsis originally stems from the historical [[fusion (linguistics)|coalescence]] of consonant clusters beginning with a nasal, both word internally and across word boundaries, i.e if a word ended in a nasal and the next word began with a stop or labial fricative, they would coalesce.

Today, many of the former final nasals have been elided, but still have an effect on the pronunciation of a following consonant, which has been [[grammaticisation|grammaticised]]. For example, the Proto-Celtic genitive plural of the definite article ''*sindoisom'' has lost its final nasal and been reduced to {{lang|ga|na}} but it now causes the eclipsis of a following consonant or the prothesis of {{vr|n-}} to a vowel. 

The cluster reductions involved in eclipsis turned nasal stops followed by a voiced stop into nasal stops, nasal stops followed by a voiceless stop into voiced plosives, nasal stops followed by a voiceless labial fricative into a voiced fricative, and words which have lost their final nasal add an {{vr|n-}} to vowel initial words.

These cluster reductions did not only occur word initially, though non-initial coalescence was never grammaticised. For example, Proto-Celtic ''*lindos'' → {{lang|sga|lind}} → {{lang|ga|linn}} "pool", and ''*kʷenkʷe'' → {{lang|sga|cóic}} → {{lang|ga|cúig}} "five".