The morphs are said to be in contrastive distribution if their meanings (functions) are different. Such morphs constitute different morphemes.
Example:
the suffixes -(e)d and -ing in the verb-forms returned, returning.
The morphs are said to be in non-contrastive distribution if their meaning (function) is the same. Such morphs constitute ‘free alternants’ (free variants) of the same morpheme.
Example:
the suffixes -(e)d and -t in the verb-forms learned, learnt.
Complementary distribution can be understood as relation of formally different morphs having the same function in different environments. Two or more morphs are said to be in complementary distribution if they have the same meaning and the difference in their form is explained by different environments. They are considered to be the allomorphs of the same morpheme, i.d. an allomorph is a linguistics term for a variant form of a morph.
Examples:
1) The plural morpheme –s. It occurs is several allomorphs depending on its phonological environment namely /-s/, /-z/, /-iz/ which stand in phonemic complementary distribution.
2) The past tense morpheme –ed occurs in several allomorphs /-id/, /-t/, /-d/.
3) The plural allomorph -en in oxen, children and the zero suffix of sheep stand in morphemic complementary distribution with the other allomorphs of the plural morpheme.
So the notion of complementary distribution helps establish the identity of grammatical elements.
As a result of the application of distributional analysis to the morphemic level, the following distributional types of morpheme are distinguished:
I) according to the degree of self-dependence (по степени самостоятельности)