Consonant mutation
Consonant mutation is the phenomenon in which a consonant in a word is changed according to its morphological and/or syntactic environment.
Mutation phenomena are found in languages around the world. The prototypical example of consonant mutation is the initial consonant mutation of all modern Celtic languages. Initial consonant mutation is found also in Japanese, Indonesian or Malay, in Southern Paiute and in several West African languages such as Fula. The Nilotic language Dholuo, spoken in Kenya, shows mutation of stem-final consonants, as does English to a small extent. Mutation of initial, medial, and final consonants is found in Modern Hebrew.

# Celtic
The Celtic languages are well known for their initial consonant mutations. The individual languages vary on the number of mutations available: Scottish Gaelic and Manx have one, Irish has two, and the Brythonic languages Welsh, Breton and Cornish each have three (but not the same three). Additionally, Irish and the Brythonic languages have so-called "mixed mutations", where a trigger will cause one mutation to some sounds and another to other sounds. The languages vary on the environments for the mutations, though some generalizations can be made. In all the languages, feminine singular nouns are mutated after the definite article, and adjectives are mutated after feminine singular nouns. In most of the languages, the possessive adjectives trigger various mutations. Following are some examples from Breton, Irish and Welsh:
Older textbooks on Gaelic sometimes refer to the c  ch mutation as "aspiration", but it is not aspiration in the technical sense of the word, and linguists prefer to speak of lenition here.
For details see the articles on the individual languages:
- Breton language: initial consonant mutations
- Cornish language: grammar
- Irish initial mutations
- Manx language: initial consonant mutations
- Scottish Gaelic: Lenition
- Welsh morphology

# Dholuo
The Dholuo language (one of the Luo languages) shows alternations between voiced and voiceless states of the final consonant of a noun stem. In the construct state (the form that means 'hill of', 'stick of', etc.) the voicing of the final consonant is switched from the absolute state. (There are also often vowel alternations that are independent of consonant mutation.)
- Template:IPA 'hill' (abs.), god (const.)
- Template:IPA 'stick' (abs.), lu (const.)
- Template:IPA 'appearance' (abs.), kit (const.)
- Template:IPA 'bone' (abs.), Template:IPA (const.)
- buk 'book' (abs.), bug (const.)
- Template:IPA 'book' (abs.), Template:IPA (const.)

# English
English has a no longer productive process of voicing stem-final fricatives, which is encountered both in noun-verb pairs and in the formation of plural nouns.
- belief - believe
- life - live
- proof - prove
- strife - strive
- thief - thieve
- baTemplate:IPA - baTemplate:IPAe
- breaTemplate:IPA - breaTemplate:IPAe
- mouTemplate:IPA (n.) - mouTemplate:IPA (vb.)
- sheaTemplate:IPA - sheaTemplate:IPAe
- wreaTemplate:IPA - wreaTemplate:IPAe
- choiTemplate:IPAe - chooTemplate:IPAe
- houTemplate:IPAe (n.) - houTemplate:IPAe (vb.)
- uTemplate:IPAe (n.) - uTemplate:IPAe (vb.)
The voicing alternation found in plural formation is losing ground in the modern language, and of the alternations listed below many speakers retain only the  pattern, which is supported by the orthography.
- knife - knives
- leaf - leaves
- self - selves
- shelf - shelves
- wharf - wharves
- wife - wives
- wolf - wolves
- baTemplate:IPA - baTemplate:IPAs
- mouTemplate:IPA - mouTemplate:IPAs
- oaTemplate:IPA - oaTemplate:IPAs
- paTemplate:IPA - paTemplate:IPAs
- youTemplate:IPA - youTemplate:IPAs
- houTemplate:IPAe - houTemplate:IPAes

# Fula
The Gombe dialect of Fula, spoken in Nigeria, shows mutation triggered by declension class. The mutation grades are Fortition and Prenasalization:
For example, the stems rim- 'free man' and Template:IPA 'person' have the following forms:
- Template:IPA (class 2), dimo (class 1), ndimon (class 6)
- Template:IPA (class 2), gimo (class 1), gimkon (class 6)

# Modern Hebrew
Modern Hebrew shows a limited set of mutation alternations, involving spirantization only. The consonants affected may be stem-initial, stem-medial, or stem-final.
For example, some verbs show mutation between tenses and conjugation classes:
- Template:IPA "he wrote", Template:IPA "he will write"
- Template:IPA "he sank" (transitive), Template:IPA "he sank" (intransitive)
Some nouns show mutation between masculine and feminine, between singular and plural, or after prepositions:
- Template:IPA "a king", Template:IPA "a queen", Template:IPA "kings", Template:IPA (spoken Hebrew, Template:IPA)
- Template:IPA "a bear" (masc.), Template:IPA "a bear" (fem.), Template:IPA "bears" (masc.), Template:IPA "bears" (fem.)
- Template:IPA "a house", Template:IPA "in a house" (in spoken Modern Hebrew: Template:IPA
But not all words have alternations:
- Template:IPA "he hacked", Template:IPA "he will hack" (diachronically, this is due to etymologyical, unmutated, )
- Template:IPA "good", Template:IPA/Template:IPA "goods"
- Template:IPA "a kibbutz", Template:IPA "in a kibbutz"
In some limited cases, initial mutation can signal adverbial status in spoken Modern Hebrew:
- Template:IPA "a request", Template:IPA "please" (spoken or written, ""), Template:IPA "please" (spoken, informal).

# Indonesian/Malay
The active form of a multisyllabic verb with an initial stop consonant or fricative consonant is formed by prefixing the verb stem with meN-, in which N stands for a nasal consonant sharing the place of articulation as the initial consonant.
- garuk  menggaruk (= to scratch), hitung  menghitung (= to count),
- beri  memberi (= to give), fitnah  memfitnah (= to falsely accuse),
- cari  mencari (= to search), dapat  mendapat (= to obtain), *jangkau  menjangkau (= to reach)
If the initial consonant is an unvoiced stop or s, it disappears, leaving only the nasal in its place.
- kandung  mengandung (= to contain or to be pregnant),
- putih  memutih (= to turn white),
- satu  menyatu (= to become one / to unite),
- tulis  menulis (= to write).
Applied to verbs starting with a vowel, the nasal consonant is realized as ng Template:IPA.
Monosyllabic verbs add an epenthetic vowel before prefixing, producing the prefix menge-.
- bor (= boring tool / drill)  mengebor (= to make a hole with drill).
Verbs starting with a nasal or approximant consonant do not add the mutant nasal at all, just me-.

# Japanese
Rendaku (meaning sequential voicing) is a mutation of the initial consonant of a non-initial component in a Japanese compound word.
Some compounds exhibiting rendaku:
nigiri + sushi  nigirizushi ("squeeze" + "sushi"  "hand-shaped sushi")
nigori + sake  nigorizake ("muddy" + "rice wine"  "unfiltered sake")
Nigori in "nigorizake" and the daku in "rendaku" are actually different readings (see On-yomi and Kun-yomi) of the same kanji Template:Unicode, because voiced and unvoiced consonants are described in Japanese as opaque and clear.

# Southern Paiute
In Southern Paiute, there are three consonant mutations, which are triggered by different word-stems. The mutations are Spirantization, Gemination, and Prenasalization:
For example, the absolutive suffix -pi appears in different forms, according to which noun stem it is suffixed to:
- movi-ppi 'nose'
- sappI-vi 'belly'
- ao-mpi 'tongue'

# Russian
In Russian, consonant mutation and alternations are a very common phenomenon during word formation, conjugation and in comparative adjectives.
The most common classes of mutations involve
- velar/postalveolar alternation Template:IPATemplate:IPA (), Template:IPATemplate:IPA (), Template:IPATemplate:IPA (): / (comparative adjective: quietquieter)
- Gain/loss of palatalization: / (adjective formation: tsar (n.)/ tsar (adj.))
Other common mutations are:
- Template:IPATemplate:IPA / Template:IPATemplate:IPA:
- Template:IPATemplate:IPA / Template:IPATemplate:IPA / Template:IPATemplate:IPA
- Template:IPATemplate:IPA ():   

# Fictional languages

## Sindarin
The Sindarin language created by J. R. R. Tolkien has mutation patterns inspired by those of Welsh. The first letter of a noun usually undergoes mutation when the noun follows a closely associated word such as an article or preposition. Thus, we get certh, rune, and i gerth, the rune.  Also, second elements of compounds and direct objects of verbs undergo mutation.

# Mutation vs. sandhi
Initial consonant mutation must not be confused with sandhi, which can refer to word-initial alternations triggered by their phonological environment, unlike mutations, which are triggered by their morphosyntactic environment. Some examples of word-initial sandhi are listed below.
- Spanish: , occurring after nasal consonants and pause, alternate with Template:IPA, occurring after vowels and liquid consonants. Example: un arco 'a boat', mi arco 'my boat'.
- Scottish Gaelic: stops in stressed syllables are voiced after nasals, e.g.  'a cat', Template:IPA gaht] 'the cat'.
- Nivkh: stops become fricatives after vowels, and fricatives become stops after other fricatives. Examples:
- Template:IPA 'neck', Template:IPA 'neck of a reindeer'
- Template:IPA 'shoot', Template:IPA 'to shoot a bear'
Sandhi effects like these (or other phonological processes) are usually the historical origin of morphosyntactically triggered mutation. For example, the English fricative mutation described above originates in an allophonic alternation of Old English, where a voiced fricative occurred between vowels (or other voiced consonants), and a voiceless one occurred initially or finally, and also when adjacent to voiceless consonants. Old English infinitives ended in -(i)an and plural nouns (of one very common declension class) ended in -as. Thus, hus 'a house' had Template:IPA, while husas 'houses' and husian 'to house' had Template:IPA.  After most endings were lost in English, and the contrast between voiced and voiceless fricatives phonemicized (largely due to the influx of French loanwords), the alternation was morphologized.