Ablaut

In linguistics, apophony (also known as ablaut, (vowel) gradation, (vowel) mutation, alternation, internal modification, stem modification, stem alternation, replacive morphology, stem mutation, internal inflection etc.) is the alternation of sounds within a word that indicates grammatical information (often inflectional).

Apophony is exemplified in English as the internal vowel alternations that produce such related words as

The difference in these vowels marks variously a difference in tense or aspect (e.g. sing/sang/sung), transitivity (rise/raise), part of speech (sing/song), or grammatical number (goose/geese).

That these sound alternations function grammatically can be seen as they are often equivalent to grammatical suffixes (an external modification). Compare the following:

The vowel alternation between i and a indicates a difference between present and past tense in the pair sing/sang. Here the past tense is indicated by the vowel a just as the past tense is indicated on the verb jump with the past tense suffix -ed. Likewise, the plural suffix -s on the word books has the same grammatical function as the presence of the vowel ee in the word geese (where ee alternates with oo in the pair goose/geese).

Consonants, too, can alternate in ways that are used grammatically. An example is the pattern in English of verb-noun pairs with related meanings but differing in voicing of a postvocalic consonant:

Most instances of apophony develop historically from changes due to phonological assimilation that are later grammaticalized (or morphologized) when the environment causing the assimilation is lost. Such is the case with English goose/geese and belief/believe.

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