complementary distribution

In linguistics, complementary distribution, as distinct from contrastive distribution and free variation, is the relationship between two different elements of the same kind, where one element is found in one set of environments and the other element is found in a non-intersecting (i.e. complementary) set of environments. It often indicates that two superficially different elements are the same linguistic unit at a deeper level. It is possible for more than two elements to be in complementary distribution with one another.