It is necessary therefore to agree that a phonemic sequence will be considered unchanged as to its morphemic segmentation if part of its environment, in one utterance, is replaced by another stretch, making another utterance; that is, given the utterance 'XY' (where 'X', 'Y', 'Z' are each phonemic sequences), if we substitute 'Z' for 'X' and obtain the utterance 'ZY', we will consider that in these two utterances the two 'Y’s' are morphemically identical. We may call 'Y' the frame in which 'X' and 'Z' are mutually substitutable. This statemenet says nothing about the morphemic content of the phonemic sequence 'Y' in any utterance other than 'XY' and 'XZ'.
- Harris (1951), a pag.159, n.6

[…] we will consider particular tentatively independent phonemic sequences as morphemic segments only if it will turn out that many of these sequences have identical relations to many other tentatively independent phonemic sequences. Given several such sequences, 'A', 'B', etc., we would accord morphemic status to 'A', 'B', 'C', if, for example, 'A', 'B', and 'C', all occur sometimes after morphemes 'D', 'E', or 'F', but never after 'G' or 'H', where 'D', 'E', and 'F', are precisely the only morphemes which occur in environment 'X—' (i.e. if 'D', 'E', 'F' constitute a distributional class as against 'G', 'H'). Meeting this criterion may thus prevent us from carrying out, in any given utterance, some of the segmentations which 12.22 [Upper Limit for Number of Morphemic Segments in an Utterance] would permit, but for which we cannot find other segments having similar distributions. It thus reduces the number of morphemes recognized in each particular utterance.
- Harris (1951), a pag.160

[…] each time a new morphemic segment is recognized, with a certain stated phonemic constituency, it becomes unnecessary to state elsewhere that the particular phonemic sequence represented by that morpheme occurs in the environment in question, while other phonemic sequences not represented by any morpheme do not occur there.
- Harris (1951), a pag.171

Phonemic sequences which contrast (i.e. occur in the same position) but are not repetitions of each other constitute different morphemes. However, those which are complementary, as are the various occurrences of '-o', do not necessarily constitute either identical or distinct morphemes.
- Harris (1951), a pag.216, n.42