A system of elements based only upon the morphemic substitutability relation would be that of morphophonemes. If we considered only one morpheme at a time, or only morphemes with identical alternations, the [f] and [v] segments of 'knife', 'knive' would be included as alternants (members) in one element, the [ph] and [p‘] above as alternants in one other, and so on. Within these morphemes these segments are both complementary and substitutable mutually. To this arrangement, we add the phonemic requirement that the segments be complementary (or freely variable) in respect to the occurrences of all other phonemic elements.
- Harris (1951), a pag.234, n.34