By free alternation is meant (a) that one cannot predict, save perhaps statistically, which form will occur in a particular instance, and (b) that the occurrence of one, rather than of the other, does not produce an utterance different in meaning.
- Hockett (2004a), Pag. 77

Two elements of the same kind (i.e. both allophones, both morphs, or the like) are in non-contrastive distribution if either (1) they are in complementary distribution, or (2) they are in partial complementation, and in those environments in which both occur, they are in free alternation.
- Hockett (2004a), Pag. 77