Before dealing with segments having complementary environments, we may consider those which occur in identical environments. If we find that, /ˌekƏ'namiks/ 'economics' and /ˌiykƏ'namiks/ 'economics' occur in completely identical environments in all cases, i.e. that a speaker, or all the speakers in our corpus, substitute one for the other in all linguistic environments (even if not in all social situations), we call these two free variants of each other. They are then morphemically equivalent, though not phonemically so.
- Harris (1951), a pag.198