The previous unit showed us that a morpheme is the smallest unit that pairs a consistent form with a consistent meaning. But when we say that the form of a morpheme is consistent, there’s still some room for variability in the form. Think back to what you know about phonology and remember that a given phoneme can show up as different allophones depending on the surrounding environment. Morphemes work the same way: a given morpheme might have more than one allomorph. Allomorphs are forms that are related to each other but slightly different, depending on the surrounding environment.

A simple example is the English word a.  It means something like “one of something, but not any particular one”, like in these examples:

a book
a skirt
a friend
a phone call

But if the word following a begins with a vowel and not a consonant, then the word a changes its form:

an apple
an ice cream cone
an iguana
an idea

The two forms a and an are slightly different in their form, but they clearly both have the same meaning. And each one shows up in a different predictable environment: a before words that start with consonants and an before words that begin with vowels.

Another example of allomorphy in English is in the plural morpheme. In written English, the form of the plural morpheme is spelled -s, as in:

carrots
books
hats
friends
apples
iguanas

But it’s spelled –es in words like:

churches
bushes
quizzes

And in fact, even in the cases where it’s spelled -s, it’s pronounced as [s] for words that end in a voiceless segment (carrots, books, cliffs) and as [z] for words that end in voiced sounds (worms, dogs, birds).  So it’s got two written forms (-s and -es) and three spoken forms ([s], [z], [ɨz]), but a consistent meaning of “more than one”.  Each form is an allomorph of the plural morpheme. Can you figure out what the relevant environment is that predicts which allomorph appears where?