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. In English 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 or in verbs whose stem ends with the alveolar stops or , such as 'hunted' or 'banded' as in verbs whose stem ends with voiceless phonemes other than , such as 'fished' as in verbs whose stem ends with voiced phonemes other than , such as 'buzzed' The "other than" restrictions above are typical for allomorphy. If the allomorphy conditions are ordered from most restrictive (in this case, after an alveolar stop) to least restrictive, the first matching case usually has precedence. Thus, the above conditions could be rewritten as follows: as or when the stem ends with the alveolar stops or as when the stem ends with voiceless phonemes as elsewhere The allomorph does not appear after stem-final although the latter is voiceless, which is then explained by appearing in that environment, together with the fact that the environments are ordered. Likewise, the allomorph does not appear after stem-final because the earlier clause for the allomorph has priority. The allomorph does not appear after stem-final voiceless phoneme because the preceding clause for the comes first. Irregular past tense forms, such as "broke" or "was/were," can be seen as still more specific cases since they are confined to certain lexical items, such as the verb "break," which take priority over the general cases listed above. Plural allomorphs The plural morpheme for regular nouns in English is typically realized by adding an s or es to the end of the noun. However, the plural morpheme actually has three different allomorphs: [s], [z], and [əz]. The specific pronunciation that