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
- sing, sang, sung, song
- rise, raise
- bind, bound
- goose, geese
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:
Present Tense Past Tense jump jumped sing sang Singular Plural book books goose geese
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:
believe belief give gift house (phonetically: [haʊz]) house (phonetically: [haʊs]) live life rive rift use (phonetically: [juz]) use (phonetically: [jus])
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.
Vowel gradation 
Singular Plural gloss vowel alternation dom dum 'field/fields' (o-u) kat kɛt 'frame/frames' (a-ɛ)
- (Bauer 2003:35)
The vowel alternation may involve more than just a change in vowel quality. In Athabascan languages, such as Navajo, verbs have series of stems where the vowel alternates (sometimes with an added suffix) indicating a different tense-aspect. Navajo vowel ablaut, depending on the verb, may be a change in vowel, vowel length, nasality, and/or tone. For example, the verb stem -kaah/-ką́ "to handle an open container" has a total of 16 combinations of the 5 modes and 4 aspects, resulting in 7 different verb stem forms (i.e. -kaah, -kááh, -kaał, -kááł, -ka’, -ká, -ką́).
Another verb stem -géésh/-gizh "to cut" has a different set of alternations and mode-aspect combinations, resulting in 3 different forms (i.e. -géésh, -gizh, -gish):
Prosodic apophony 
tone alternation đây "here" đấy "there" (ngang tone-sắc tone) bây giờ "now" bấy giờ "then" (ngang tone-sắc tone) kia "there" kìa "yonder" (ngang tone-huyền tone) cứng "hard" cửng "(to) have an erection" (sắc tone-hỏi tone)
- (Nguyễn 1997:42-44)
[ɡuːr] "stone" [ɡur] "stones" [dy] "two (masculine)" [dyː] "two (feminine)"
- (Asher 1994:1719)
English has alternating stress patterns that indicate whether related words are nouns (first syllable stressed) or verbs (second syllable stressed):
noun verb pérvert pervért ínsult insúlt pérmit permít cónvict convíct récord recórd
Prosodic alternations are sometimes analyzed as not as a type of apophony but rather as prosodic affixes, which are known, variously, as suprafixes, superfixes, or simulfixes.
Consonant apophony 
Consonant alternation is commonly known as consonant mutation or consonant gradation. Bemba indicates causative verbs through alternation of the stem-final consonant. Here the alternation involves spirantization and palatalization:
Intransitive Verb Causative Verb luba "to be lost" lufya "to cause to be lost" koma "to be deaf" komya "to cause to be deaf" pona "to fall" ponya "to cause to fall" enda "to walk" endesha "to cause to walk" lunga "to hunt" lunsha "to cause to hunt" kula "to grow" kusha "to cause to grow"
- (Kula 2000:174)
Celtic languages are well known for their initial consonant mutations.
Indo-European linguistics 
Indo-European ablaut 
In Indo-European linguistics, ablaut is the vowel alternation that produces such related words as sing, sang, sung, and song. The difference in the vowels results from the alternation (in the Proto-Indo-European language) of the vowel e with the vowel o or with no vowel.
Imperative Preterite Past
vowel alternation swim swam swum (i-a-u)
fall fell fallen (a-e-a)
drive drove driven (i-o-i)
Ablaut versus umlaut 
In Indo-European linguistics, umlaut is the vowel alternation that produces such related words as foot and feet or strong and strength. The difference in the vowels results from the influence (in Proto-Germanic or a later Germanic language) of an i or y (which has since been lost) on the vowel which (in these examples) becomes e.
To cite another example of umlaut, some English weak verbs show umlaut in the present tense.
vowel alternation bring brought (i-ou)
In Indo-European historical linguistics the terms ablaut and umlaut refer to different phenomena and are not interchangeable. 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 an Indo-European perspective, it typically appears as a variation between o, e, and no vowel, although various sound changes result in different vowel alternations appearing in different daughter languages. Umlaut, meanwhile, is a process that is particular to the Germanic languages and refers to a variation between back vowels and front vowels that was originally phonologically predictable, and was caused by the presence of an /i/ or /j/ in the syllable following the modified vowel.
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. When taking a synchronic perspective on modern languages, however, both processes appear very similar. For example, the alternations seen in sing/sang/sung and foot/feet both appear to be morphologically conditioned (e.g. the alternation appears in the plural or past tense, but not the singular or present tense) and phonologically unpredictable.
By analogy, descriptive linguists discussing synchronic grammars sometimes employ the terms ablaut and umlaut imprecisely, 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). Ambiguity can of course be avoided by using alternative terms (apophony, gradation, alternation, internal modification for ablaut, vowel harmony for umlaut) for the broader sense of the words.
Stem alternations and other morphological processes 
Singular Plural Buch "book" Bücher "books" Haus "house" Häuser "houses"
Here the singular/plural distinction is indicated through ablaut and additionally by a suffix -er in the plural form. 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:
child (singular) // children (plural) /ˈtʃɪldrən/ drive (imperative) // driven (past participle) /ˈdrɪvən/
Positive Negative hilhali "I'm dancing" akhi'lho "I'm not dancing"
The nonconcatenative root-and-pattern morphology of the Afro-Asiatic languages is sometimes described in terms of apophony. The alternation patterns in many of these languages is quite extensive involving vowels and consonant gemination (i.e. doubled consonants). The alternations below are of Modern Standard Arabic (the symbol ⟨ː⟩ indicates gemination on the preceding consonant):
word gloss alternation pattern katab "to write" (a - a) kataba "he wrote" (a - a - a) kaatab "to correspond with" (aː - a) kattab "to cause to write" (a - ːa) kuttib "to be caused to write" (u - ːi) kitaab "book" (i - aː) kutub "books" (u - u) kaatib "writer" (aː - i) kuttaab "writers" (u - ːaː)
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). Some theoretical perspectives call up the notion of morphological templates or morpheme "skeletons".
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)". Many would consider this type of analysis for English to be less desirable as this type of infixal morphology is not very prevalent throughout English and the morphemes -oo- and -ee- would be exceedingly rare.
Replacive morphemes 
Another analytical perspective on sound alternations treats the phenomena not as merely alternation but rather a "replacive" morpheme that replaces part of a word. In this analysis, the alternation between goose/geese may be thought of as goose being the basic form where -ee- is a replacive morpheme that is substituted for oo.
- goose → g-ee-se
This usage of the term morpheme (which is actually describing a replacement process, and not a true morpheme), however, is more in keeping with Item-and-Process models of morphology instead of Item-and-Arrangement models.
English ablaut-motivated compounding 
Here the words are formed by a reduplication of a base and an alternation of the internal vowel.
Some examples in Japanese:
- kasa-koso (rustle)
- gata-goto (rattle)
See also 
- Alternation (linguistics)
- Consonant mutation
- Morphology (linguistics)
- Nonconcatenative morphology
- References for ablaut
- Anderson, Stephen R. (1985). Inflectional morphology. In T. Shopen (Ed.), Language typology and syntactic description: Grammatical categories and the lexicon (Vol. 3, pp. 150–201). Cambridge: Cambridge University Press. (Especially section 1.3 "Stem modifications").
- Asher, R. E. (Ed.). (1994). The Encyclopedia of language and linguistics. Oxford: Pergamon Press. ISBN 0-08-035943-4.
- Bauer, Laurie. (2003). Introducing linguistic morphology (2nd ed.). Washington, D.C.: Georgetown University Press.
- Bauer, Laurie. (2004). A glossary of morphology. Washington, D.C.: Georgetown University Press.
- Hamano, Shoko. (1998). The Sound-Symbolic System of Japanese. CSLI Publications,Stanford.
- Haspelmath, Martin. (2002). Understanding morphology. London: Arnold.
- Kula, Nancy C. (2000). The phonology/morphology interface: Consonant mutations in Bemba. In H. de Hoop & T. van der Wouden (Eds.), Linguistics in the Netherlands 2000 (pp. 171–183). Amsterdam: John Benjamins.
- Nguyễn, Đình-Hoà. (1997). Vietnamese: Tiếng Việt không son phấn. Amsterdam: John Benjamins Publishing Company. ISBN 1-55619-733-0.
- Sapir, Edward. (1921). Language: An introduction to the study of speech. New York: Harcourt, Brace & Co.
- Spencer, Andrew; & Zwicky, Arnold M. (Eds.). (1998). The handbook of morphology. Oxford: Blackwell.
- Young, Robert W., & Morgan, William, Sr. (1987). The Navajo language: A grammar and colloquial dictionary (rev. ed.). Albuquerque: University of New Mexico Press. ISBN 0-8263-1014-1.