[…] morphophonemes are linguistically useful in that they indicate a special relation among phonemes. For example, […] the sounds (in morphemes) which are usually indistinguishable from each other for the speakers of the language are those which are members both of one phoneme and of one morphophoneme.
- Harris (1951), a pag.226, n.17

When it is possible to differentiate phonemically between the morphemes in which an alternation occurs and those in which it does not, and between the environments of the one member of the alternation as against the environments of the other, […] it is not necessary to introduce a new morphophonemic symbol, since both the morphemes and the environments can be identified by means of their phonemes. In such cases we merely define a morphophoneme, symbolized by the characteristic phonemes of the morpheme, in such a way that it represents in each environment of the morpheme the phonemes which occur in that environment.
- Harris (1951), a pag.227

In cases where a given alternation occurs in all morphemes which have a particular phonemic feature and in all occurrences of a phonemically stateable environment […] we can consider all the members to consist of morphophonemes based on the phonemically most complicated or arbitrary member. Instead of having a Kota morpheme with two members, phonemically /katy/ and /ka/ ‘knife’, we have a morpheme with one member, morphophonemically /katy/; the morphophonemes are identical with the phonemes of the first member, but the last two morphophonemes represent phonemic zero when they occur before /t/; otherwise they represent phonemic /ty/.
- Harris (1951), a pag.228

It is also not necessary to introduce a new morphophonemic symbol when it is possible to differentiate phonemically between the morphemes in which an alternation occurs and those in which it does not, or when the environments in which the morphemes have their special member consist of a small number of morphemes […]. In such cases it may be sufficient to define a morphophoneme, symbolized by the interchange of phonemes which constitutes the alternation in question, and to say that that morphophoneme is part of the morphophonemic and phonemic composition of the stated environmental morphemes. Thus, the morpheme 'ity' […] has the phonemic form /k → s; ey → æ; Ətiy/, the /k → s/ being understood to apply only to any preceding /k/, and /ey → æ/ to any preceding /eyC/. For the sake of abbreviation, a new symbol, say / ˘/, may be defined to represent these changes before 'ity', so that the composition of that suffix becomes / ˘Ətiy/.
- Harris (1951), a pag.229

Each morphophone 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. A morphophoneme is thus a class of phoneme-length segments, the same segments that we had grouped into phonemes, except that into one morphophoneme we group segments which are complementary within one morpheme (holding the morpheme constant), while into phonemes we grouped segments which were complementary without regard to morpheme constancy.
- Harris (1951), a pag.232, n.30

There are two types of situation in which morphophonemes could be used even though the phonemes they represent, and the morphemic segments in which they occur, are not mutually complementary in environment. One is the free variation among phonemically distinct morphemic segments, as in the case of 'economics' […]. We may define a morphophoneme /E/ freely representing /e/ sometimes and /iy/ at other times, and then write /ˌEkƏ'namics/ but /ˌele'mentƏl/. This is more useful if there are many morphemes in which the identical free variation occurs. The other is the intermittently present pause or other feature […]. If we do not wish to recognize these elements which can be observed in repetitions of an utterance rather in a single pronounciation of it, we can define a morphophoneme which sometimes represents the feature and sometimes represents its absence. Each morpheme would then have the same morphophonemic constitution in all its occurrences, even though its phonemic constitution varies freely.
- Harris (1951), a pag.232, n.31

The interchange of phonemes or components in corresponding sections of the variant members of each morpheme can then constitute a class called a morphophoneme.
- Harris (1951), a pag.362

[…] morphophonemes are classes of corresponding segments in stretches of speech which are equivalent in their morphemic composition.
- Harris (1951), a pag.362, n.3