Lenition

In linguistics, lenition is a sound change that alters consonants, making them "weaker" in some way. The word lenition means 'softening' or 'weakening'. Lenition can happen both synchronically and diachronically. Lenition can involve such changes as voicing a voiceless consonant, causing a consonant to relax occlusion, to lose its place of articulation, or even causing a consonant to disappear entirely.
An example of synchronic lenition is found in most varieties of American English, in the form of tapping: the of a word like wait is pronounced as the more sonorous in the related form waiting. Some varieties of Spanish show debuccalization of to at the end of a syllable, so that a word like estamos "we are" is pronounced. An example of diachronic lenition can be found in the Romance languages, where the of Latin patrem has become in Italian and Spanish padre, while in Catalan pare, French père and Portuguese pai historical has disappeared completely.
In some languages, lenition has been grammaticalized into a consonant mutation, which means it is no longer triggered by its phonological environment but is now governed by its syntactic or morphological environment. For example, in Welsh, the word cath "cat" begins with the sound, but after the definite article y, the changes to : "the cat" in Welsh is y gath. This was historically due to intervocalic lenition, but in the plural, lenition does not happen, so "the cats" is y cathod, not *y gathod. The change of to in y gath is thus caused by the syntax of the phrase, not by the modern phonological position of the consonant.
The opposite of lenition, fortition, a sound change that makes a consonant "stronger", is less common, but Breton and Cornish have "hard mutation" forms which represent fortition.

Types

Lenition involves changes in manner of articulation, sometimes accompanied by small changes in place of articulation. There are two main lenition pathways: opening and sonorization. In both cases, a stronger sound becomes a weaker one. Lenition can be seen as a movement on the sonority hierarchy from less sonorous to more sonorous, or on a strength hierarchy from stronger to weaker.
In examples below, a greater-than sign indicates that one sound changes to another. The notation > means that changes to.
The sound change of palatalization sometimes involves lenition.
Lenition includes the loss of a feature, such as deglottalization, in which glottalization or ejective articulation is lost: or >.
The tables below show common sound changes involved in lenition. In some cases, lenition may skip one of the sound changes. The change voiceless stop > fricative is more common than the series of changes voiceless stop > affricate > fricative.

Opening

In the opening type of lenition, the articulation becomes more open with each step. Opening lenition involves several sound changes: shortening of double consonants, affrication of stops, spirantization or assibilation of stops or affricates, debuccalization, and finally elision.

or >
>
or >
> ; >
,,,, > ∅

geminated stop	→	stop	→	affricate	→	fricative	→	placeless approximant	→	no sound
original sound	→	degemination	→	affrication	→	spirantization	→	debuccalization	→	elision
	→		→		→		→		→
	→		→		→		→		→
	→		→		→		→		→
	→		→		→		→		→
	→		→		→		→		→

Sonorization

The sonorization type involves voicing. Sonorizing lenition involves several sound changes: voicing, approximation, and vocalization.

Sonorizing lenition occurs especially often intervocalically. In this position, lenition can be seen as a type of assimilation of the consonant to the surrounding vowels, in which features of the consonant that are not present in the surrounding vowels are gradually eliminated.
Some of the sounds generated by lenition are often subsequently "normalized" into related but cross-linguistically more common sounds. An example would be the changes → → and → →. Such normalizations correspond to diagonal movements down and to the right in the above table. In other cases, sounds are lenited and normalized at the same time; examples would be direct changes → or →.

Vocalization

is a subtype of the sonorization type of lenition. It has two possible results: a velar approximant or back vowel, or a palatal approximant or front vowel. In French, l-vocalization of the sequence resulted in the diphthong, which was monophthongized, yielding the monophthong in Modern French.

lateral approximant	→	semivowel	→	vowel
	→		→
	→		→

Mixed

Sometimes a particular example of lenition mixes the opening and sonorization pathways. For example, may spirantize or open to, then voice or sonorize to.
Lenition can be seen in Canadian and American English, where and soften to a tap when not in initial position and followed by an unstressed vowel. For example, both rate and raid plus the suffix -er are pronounced. The Italian of Central and Southern Italy has a number of lenitions, the most widespread of which is the deaffrication of to between vowels: post-pausal cena 'dinner' but post-vocalic la cena 'the dinner'; the name Luciano, although structurally, is normally pronounced. In Tuscany, likewise is realized between vowels, and in typical speech of Central Tuscany, the voiceless stops in the same position are pronounced respectively, as in → 'the house', → 'hole'.

Effects

Diachronic

lenition is found, for example, in the change from Latin into Spanish, in which the intervocalic voiceless stops first changed into their voiced counterparts, and later into the approximants or fricatives : vita > vida, lupa > loba, caeca > ciega, apotheca > bodega. One stage in these changes goes beyond phonetic to have become a phonological restructuring, e.g. > . The subsequent further weakening of the series to phonetic, as in is diachronic in the sense that the developments took place over time and displaced as the normal pronunciations between vowels. It is also synchronic in an analysis of as allophonic realizations of : illustrating with, 'wine' is pronounced after pause, but with intervocalically, as in 'of wine'; likewise, →.
A similar development occurred in the Celtic languages, where non-geminate intervocalic consonants were converted into their corresponding weaker counterparts through lenition, and voiceless stops became voiced. For example, Indo-European intervocalic -t- in teu̯teh₂ "people" resulted in Proto-Celtic, Primitive Irish tōθā, Old Irish and ultimately debuccalisation in most Irish and some Scottish dialects to, shift in Central Southern Irish to, and complete deletion in some Modern Irish and most Modern Scots Gaelic dialects, thus.
An example of historical lenition in the Germanic languages is evidenced by Latin-English cognates such as pater, tenuis, cornu vs. father, thin, horn. The Latin words preserved the original stops, which became fricatives in old Germanic by Grimm's law. A few centuries later, the High German consonant shift led to a second series of lenitions in Old High German, chiefly of post-vocalic stops, as evidenced in the English-German cognates ripe, water, make vs. reif, Wasser, machen.
Although actually a much more profound change encompassing syllable restructuring, simplification of geminate consonants as in the passage from Latin to Spanish such as cuppa > 'cup' is often viewed as a type of lenition.

Synchronic

Allophonic

All varieties of Sardinian, with the sole exception of Nuorese, offer an example of sandhi in which the rule of intervocalic lenition applying to the voiced series extends across word boundaries. Since it is a fully active synchronic rule, lenition is not normally indicated in the standard orthographies.
A series of synchronic lenitions involving opening, or loss of occlusion, rather than voicing is found for post-vocalic in many Tuscan dialects of Central Italy. Stereotypical Florentine, for example, has the of as casa 'house' in a post-pause realization, in casa 'in house' post-consonant, but la casa 'the house' intervocalically. Word-internally, the normal realization is also : buco 'hole' →.

Grammatical

In the Celtic languages, the phenomenon of intervocalic lenition historically extended across word boundaries. This explains the rise of grammaticalised initial consonant mutations in modern Celtic languages through the loss of endings. A Scottish Gaelic example would be the lack of lenition in am fear and lenition in a' bhean . The following examples show the development of a phrase consisting of a definite article plus a masculine noun compared with a feminine noun taking the ending -a. The historic development of lenition in those two cases can be reconstructed as follows:
Synchronic lenition in Scottish Gaelic affects almost all consonants. Changes such as to involve the loss of secondary articulation; in addition, → involves the reduction of a trill to a tap. The spirantization of Gaelic nasal to is unusual among forms of lenition, but it is triggered by the same environment as more prototypical lenition. The orthography shows that by inserting an h.