With the recognition of the uniquely occurring morphs and their association in sets of identical allomorphs, we have made a good start toward moving up the ladder of linguistic structure to the next level. One thing seems certain even this early: we shall find a much greater number and variety of units on this level than we did on the phonemic level. The number of different combinations that can be made from 33 segmental phonemes is very large indeed. In fact, we can be sure that no matter how many allomorphs we may discover, they will be only a small percentage of the total mathematical possibility. It is here, in fact, that the great diversity and adaptability of language begins to show itself. And it is here that we must give up the hope of being as exhaustive in our treatment as we were in our discussions of phonetics and phonemics, that we cannot hope to list all the allomorphs in English. Instead we can deal only with representative types and illustrations of morphemic structure. [...]
Definition: A morpheme is a group of allomorphs that are semantically similar and in complementary distribution.
As we have suggested in the title of this chapter, morphemes are the building blocks out of which the meaningful utterances of speech are put together. A morpheme is a group of allomorphs, each of which is a combination of phonemes; but, as we pointed out in the first chapter, in structure of the kind the language shares with many other natural and man-made phenomena, the whole is more than the sum of all its parts. When phonemes are organized into an allomorph, meaning is added to make a new thing, just as when hydrogen and oxygen are organized into water, a substance emerges that has new and different qualities which could not have been guessed from a knowledge of the qualities of its components. From here on up the ladder of increasingly complex linguistic structure, we shall observe increasingly complex and precise indications of meaning, for after all it is to communicate meanings that language had been created, therefore, morphemes, the smallest structural units possessing meaning, occupy a key position in linguistic structure. They are the fundamental building blocks out of which everything we say is built.