Two or more parallel sets of tentative phonemes, such as the /ay, ey/ and /Ay, Ey/ sets, cannot be combined into one set because they represent distinct segments in identical environments. However, they could be combined if there were a technique for altering the environment of one of the sets so that its environment should no longer be identical with that of the other: if every /Ay/ and /Ey/ had some environmental difference as against every /ay/ and /ey/, then /ay/ would be complementary to /Ay/ and the two could be put into one phoneme, and so for /ey/ and /Ey/. Any such alteration would have to be controlled and reversible; otherwise writing which included this alteration would no longer be a one-one representation of the descriptively relevant features.
- Harris (1951), Pag. 79