· "The indefinite articleis a good example of a morpheme with more than one allomorph. It is realised 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)

· Morphs and Allomorphs

"[W]hen 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 word . . . as the morph that produces the 'irregular' plural form men."

(George Yule, The Study of Language, 4th ed. Cambridge Univ. Press, 2010)