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 any alternation within a word that indicates grammatical information (often inflectional). Description Apophony is exemplified in English as the internal vowel alternations that produce such related words as sng, sng, sng, sng bnd, bnd bld, bld brd, brd dm, dm fd, fd l, l rse, rse, rsen wve, wve ft, ft gse, gse tth, tth 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 breath/breathe. Types Apophony may involve various types of alternations, including vowels, consonants, prosodic elements (such as tone, syllable length), and even smaller features, such as nasality (on vowels). The sound alternations may be used inflectionally or derivationally. The particular function of a given alternation will depend on the language. Vowel gradation Apophony often involves vowels. Indo-European ablaut (English sing-sang) and Germanic umlaut (goose-geese), mentioned above, are well attested examples. Another example is from Dinka: When it comes to plurals, a common vowel alteration in Assyrian Neo-Aramaic (a Semitic language) is shifting the ɑ sound to e as shown in this table: The vowel alternation may involve more than just a change in vowel quality. In Athabaskan languages, such as Navajo, verbs have series of stems where