Allomorph

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. A possible set of assimilations is:

• as or (schwi) 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 (that is, listed in order of priority). 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 a plural morpheme takes on is determined by a set of morphological rules such as the following:

• assume that the basic form of the plural morpheme, /-z/, is [-z] ("bags" /bægz/)
• the morpheme /-z/ becomes [-əz] by inserting an [ə] before [-z] when a noun ends in a sibilant ("buses" /bʌsəz/)
• change the morpheme /-z/ to a voiceless [-s] when a noun ends in a voiceless sound ("caps" /kæps/)

Negative allomorphs

In English, the negative prefix in- has three allomorphs: [ɪn-], [ɪŋ-], and [ɪm-]. The phonetic form that the negative morpheme /ɪn-/ uses is determined by a set of morphological rules; for example:

• the negative morpheme /ɪn-/ becomes [ɪɱ-] when preceding a labiodental fricative ("invisible"/ɪɱˈvɪzᵻbl̴/)
• the morpheme /ɪn-/ becomes [ɪŋ-] before a velar consonant ("incongruous" /ɪŋˈkɔŋgruəs/)
• the morpheme /ɪn-/ becomes [ɪm-] before a bilabial consonant ("improper" /ɪmˈprɔpər/)