Allomorph Word Forms and Sounds

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In phonology, an allomorph is a variant form of a morpheme. (A morpheme is the smallest unit of a language.) For example, the plural in English has three different morphs, making plural an allomorph, because there are alternatives. Not all plurals are formed in the same way; they're made in English with three different morphs: /s/, /z/, and [əz], as in kicks, cats, and sizes, respectively. 

For example, "when we find a group of different morphs, all versions of one morpheme, we can use the prefix allo- ( = one of a closely related set) and describe them as allomorphs of that morpheme.

"Take the morpheme 'plural.' Note that it can be attached to a number of lexical morphemes to produce structures like 'cat + plural,' 'bus + plural,' 'sheep + plural,' and 'man + plural.' In each of these examples, the actual forms of the morphs that result from the morpheme 'plural' are different. Yet they are all allomorphs of the one morpheme. So, in addition to /s/ and /əz/, another allomorph of 'plural' in English seems to be a zero-morph because the plural form of sheep is actually 'sheep + ∅.' When we look at 'man + plural,' we have a vowel change in the the morph that produces the 'irregular' plural form men." (George Yule, "The Study of Language," 4th ed. Cambridge University Press, 2010)

Past Tense Allomorphs

Past tense is another morpheme that has multiple morphs and is thus an allomorph. When you form the past tense, you add the sounds /t/, /d/, and /əd/ to words to put them in past tense, such as in talked, grabbed, and wanted, respectively.

"Completely arbitrary allomorphs, such as English went (go + past tense), are relatively rare in the lexicon, and occur almost exclusively with a few very frequent words. This unpredictable kind of allomorphy is called suppletion." (Paul Georg Meyer, "Synchronic English Linguistics: An Introduction," 3rd ed. Gunter Narr Verlag, 2005)

Pronunciation Can Change

Depending on the context, allomorphs can vary in shape and pronunciation without changing meaning, and the formal relation between phonological allomorphs is called an alternation. "[A]n underlying morpheme can have multiple surface level allomorphs (recall that the prefix 'allo' means 'other'). That is, what we think of as a single unit (a single morpheme) can actually have more than one pronunciation (multiple allomorphs)...We can use the following analogy: phoneme: allophone = morpheme: allomorph." (Paul W. Justice, "Relevant Linguistics: An Introduction to the Structure and Use of English for Teachers," 2nd ed. CSLI, 2004)

For example, "[t]he indefinite article is a good example of a morpheme with more than one allomorph. It is realized by the two forms a and an. The sound at the beginning of the following word determines the allomorph that is selected. If the word following the indefinite article begins with a consonant, the allomorph a is selected, but if it begins with a vowel the allomorph an is used instead...

"[A]llomorphs of a morpheme are in complementary distribution. This means that they cannot substitute for each other. Hence, we cannot replace one allomorph of a morpheme by another allomorph of that morpheme and change meaning." (Francis Katamba, "English Words: Structure, History, Usage," 2nd ed. Routledge, 2004)

More on the Term Itself

 The term's adjectival use is allomorphic. Its etymology derives from the Greek,  "other" + "form."

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Nordquist, Richard. "Allomorph Word Forms and Sounds." ThoughtCo, Aug. 26, 2020, Nordquist, Richard. (2020, August 26). Allomorph Word Forms and Sounds. Retrieved from Nordquist, Richard. "Allomorph Word Forms and Sounds." ThoughtCo. (accessed March 23, 2023).