We may find that two segments are almost always complementary in environment: e.g. [s] may occur only before [a, o, u], [š] only before [i, e]. We would then phonemicize [sa] as /sa/, and [ši] as /si/, saying that [š] is the member of /s/ before [i, e].
- Harris (1951), a pag.93, n.7

Each morphophoneme is itself a class of complementary phonemic segments. The segments represented by the morphophoneme are those which occur in a particular position in all the members of a particular morpheme. They are complementary, since for each environment of the unit, its morphophonemes indicate the segments in the corresponding parts of that member of the morpheme which occurs in that environment.
- Harris (1951), a pag.232