Allomorphs are phonologically distinct variants of the same morpheme. By phonologically distinct, we mean that they have similar but not identical sounds. And when we say that they are variants of the same morpheme, we mean that these slightly different-sounding sets of forms share the same meaning or function. For example, the negative prefix in- in English is often pronounced in- (as in intolerable), but it is also sometimes pronounced im- or il- (impossible, illegal), as English spelling shows. Since all of these forms still mean ‘negative’, and they all attach to adjectives in the same way, we say that they are allomorphs of the negative prefix. Another example you’ve already seen is the regular past tense in English. Although the regular past tense in English is always spelled -ed, it is sometimes pronounced [t] (packed), sometimes [d] (bagged), sometimes [əd] (waited).1 Still all three phonological variants still designate the past tense. Similarly, the plural morpheme in Turkish sometimes appears as -lar and sometimes as -ler, so Turkish has two allomorphs of the plural morpheme.

in many cases, it is phonologically predictable which allomorph appears where; sometimes, however, which allomorph appears with a particular base is unpredictable. For example, we will see that it is usually possible to predict the form of the regular allomorphs of the English past tense morpheme, but there are quite a few verbs whose past tenses are irregular (for example, sang, flew, bought).