Apophony
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.
In Athabaskan languages, such as Navajo, verbs have series of stems where the vowel alternates (sometimes with an added suffix) indicating a different tense-aspect.
In Indo-European linguistics, ablaut is the vowel alternation that produces such related words as sing, sang, sung, and song.
In Indo-European linguistics, umlaut is the vowel fronting that produces such related words as foot > feet or strong > strength.
Ablaut is a process that dates back to Proto-Indo-European times, occurs in all Indo-European languages, and refers to (phonologically) unpredictable vowel alternations of a specific nature.
From a diachronic (historical) perspective, the distinction between ablaut and umlaut is very important, particularly in the Germanic languages, as it indicates where and how a specific vowel alternation originates.
It is also important when taking a synchronic (descriptive) perspective on old Germanic languages such as Old English, as umlaut was still a very regular and productive process at the time.
By analogy, descriptive linguists discussing synchronic grammars sometimes employ the terms ablaut and umlaut, using ablaut to refer to morphological vowel alternation generally (which is unpredictable phonologically) and umlaut to refer to any type of regressive vowel harmony (which is phonologically predictable).
English also displays similar forms with a -ren suffix in the plural and a -en suffix in the past participle forms along with the internal vowel alternation: Chechen features this as well: A more complicated example comes from Chickasaw where the positive/negative distinction in verbs displays vowel ablaut along with prefixation (ak-) and infixation (-'-): The nonconcatenative morphology of the Afroasiatic languages is sometimes described in terms of apophony.
The alternations below are of Modern Standard Arabic, based on the root k–t–b 'write' (the symbol ⟨ː⟩ indicates gemination on the preceding consonant): Other analyses of these languages consider the patterns not to be sound alternations, but rather discontinuous roots with discontinuous affixes, known as transfixes (sometimes considered simulfixes or suprafixes).
It would also be possible to analyze English in this way as well, where the alternation of goose/geese could be explained as a basic discontinuous root g-se that is filled out with an infix -oo- "(singular)" or -ee- "(plural)".
In many Turkic languages the vowel pattern is low to high, as in Turkish çak-çuk (which follows the English patten) and fan-fin (which contravenes it).
