An allomorph is a linguistics term for a variant form of a morpheme. The concept occurs when a unit of meaning can vary in sound (phonologically) without changing meaning. It is used in linguistics to explain the comprehension of variations in sound for a specific morpheme.
For example, in English, a past tense morpheme is -ed. It occurs in several allomorphs depending on its phonological environment, assimilating voicing of the previous segment or inserting a schwa when following an alveolar stop:
Notice the "other than" restrictions above. This is a common fact about allomorphy: if the allomorphy conditions are ordered from most restrictive (in this case, after an alveolar stop) to least restrictive, then the first matching case usually "wins". Thus, the above conditions could be re-written as follows:
The fact that the /t/ allomorph does not appear after stem-final /t/, despite the fact that the latter is voiceless, is then explained by the fact that /əd/ appears in that environment, together with the fact that the environments are ordered. Likewise, the fact that the /d/ allomorph does not appear after stem-final /d/ is because the earlier clause for the /əd/ allomorph takes priority; and the fact that the /d/ allomorph does not appear after stem-final voiceless phonemes is because the preceding clause for the /t/ takes priority.
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, like the verb "break"), which therefore take priority over the general cases listed above.
There are three allomorphs of the stem: /vaːk/, /vaːʧ/ and /vaːg/. The allomorphs are conditioned by the particular case-marking suffixes.
The form of the stem /vaːk/, found in the nominative singular and locative plural, is the etymological form of the morpheme. Pre-Indic palatalization of velars resulted in the variant form /vaːʧ/, which was initially phonologically conditioned. This conditioning can still be seen in the Locative Singular form, where the /ʧ/ is followed by the high front vowel /i/.
But subsequent merging of /e/ and /o/ into /a/ made the alternation unpredictable on phonetic grounds in the Genitive case (both Singular and Plural), as well as the Nominative Plural and Instrumental Singular. Hence, this allomorphy was no longer directly relatable to phonological processes.
Phonological conditioning also accounts for the /vaːg/ form found in the Instrumental Plural, where the /g/ assimilates in voicing to the following /bʱ/.