In linguistics, an allomorph is a variant phonetic form of a morpheme, or, a unit of meaning that varies in sound and spelling without changing the meaning. The term allomorph describes the realization of phonological variations for a specific morpheme. The different allomorphs that a morpheme can become are governed by morphophonemic rules. These phonological rules determine what phonetic form, or specific pronunciation, a morpheme will take based on the phonological or morphological context in which they appear.
English has several morphemes that vary in sound but not in meaning, such as past tense morphemes, plural morphemes, and negative morphemes.
Past tense allomorphs
For example, an English past tense morpheme is -ed, which occurs in several allomorphs depending on its phonological environment by assimilating the voicing of the previous segment or the insertion of a schwa after an alveolar stop:
*as [əd] or