One way to treat differences in inflectional morphemes is by proposing variation in morphological realization rules. In order to do this, we draw an analogy with some processes already noted in phonology (Chapter 4). Just as we treated phones as the actual phonetic realization of phonemes, so we can propose morphs as the actual forms used to realize morphemes. For example, the form cats consists of two morphs, cat +-s, realizing a lexical morpheme and an inflectional morpheme (“plural”). The form buses also consists of two morphs (bus + -es), realizing a lexical morpheme and an inflectional morpheme (“plural”). So there are at least two different morphs (-s and -es, actually /s/ and /əz/) used to realize the inflectional morpheme “plural.” Just as we noted that there were “allophones” of a particular phoneme, so we can recognize the existence of allomorphs of a particular morpheme. That is, when 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 + ø.” When we look at “man + plural,” we have a vowel change in the word (æ → ɛ) as the morph that produces the “irregular” plural form men.

There are a number of other morphological processes at work in a language like English, such as those involved in the range of allomorphs for the morpheme “past tense.” These include the common pattern in “walk + past tense” that produces walked and also the special pattern that takes “go + past tense” and produces the “irregular” past form went.