Allomorphs are different forms of the same morpheme, or basis unit of meaning. These can be different pronunciations or different spellings.

Example
There are three allomorphs of the morpheme -s in English. Compare the sound of the -s in ‘cats', ‘dogs' and ‘foxes'.

In the classroom
Certain allomorphs are difficult for learners to produce correctly, for example the allomorphs of the -ed regular past morpheme, which learners often do not produce correctly until higher levels. Recognition activities such as sorting words into groups according to pronunciation are useful to raise awareness.