In linguistics, an alternation is the phenomenon of a morpheme exhibiting variation in its phonological realization. Each of the various realizations is called an alternant. The variation may be conditioned by the phonological, morphological, and/or syntactic environment in which the morpheme finds itself.
Alternations provide linguists with data that allow them to determine the allophones and allomorphs of a language's phonemes and morphemes and to develop analyses determining the distribution of those allophones and allomorphs.
An example of a phonologically conditioned alternation is the Englishplural marker commonly spelled s or es. This morpheme is pronounced /s/, /z/, or /ᵻz/,[note 1] depending on the nature of the preceding sound.
If the preceding sound is a sibilant consonant (one of /s/, /z/, /ʃ/, /ʒ/), or an affricate (one of /tʃ/, /dʒ/), the plural marker takes the form /ᵻz/. Examples:
mass/ˈmæs/, plural masses/ˈmæsᵻz/
fez/ˈfɛz/, plural fezzes/ˈfɛzᵻz/
mesh/ˈmɛʃ/, plural meshes/ˈmɛʃᵻz/
mirage/mɪˈrɑːʒ/, plural mirages/mɪˈrɑːʒᵻz/
church/ˈtʃɜːrtʃ/, plural churches/ˈtʃɜːrtʃᵻz/
bridge/ˈbrɪdʒ/, plural bridges/ˈbrɪdʒᵻz/
Otherwise, if the preceding sound is voiceless, the plural marker takes the likewise voiceless form /s/. Examples:
mop/ˈmɒp/, plural mops/ˈmɒps/
mat/ˈmæt/, plural mats/ˈmæts/
pack/ˈpæk/, plural packs/ˈpæks/
cough/ˈkɒf/, plural coughs/ˈkɒfs/
myth/ˈmɪθ/, plural myths/ˈmɪθs/
Otherwise, the preceding sound is voiced, and the plural marker takes the likewise voiced form /z/.
French has an example of morphologically conditioned alternation. The feminine form of many adjectives ends in a consonant sound that is missing in the masculine form. In spelling, the feminine ends in a silent e, while the masculine ends in a silent consonant letter:
^The vowel of the inflectional suffix -⟨es⟩ may belong to the phoneme of either /ɪ/ or /ə/ depending on dialect, and ⟨ᵻ⟩ is a shorthand for "either /ɪ/ or /ə/". This usage of the symbol is borrowed from the Oxford English Dictionary.