In linguistics, apophony (also known as ablaut, gradation, 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, bind/bound), 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).