Some alternations of segments are unique to a particular morpheme. E.g. the morpheme {s} ‘plural’ has a member 'en' after {ox}. No other morpheme has, in a special position comparable to that of occurring after {ox}, a special member having the same relation to the other members of that morpheme which 'en' has to the other members of {s}. E.g. the unit {‘s} ‘possessive’ has no special member after {ox} or after any other single morpheme of that class.
- Harris (1951), a pag.220

Some alternations occur analogously in several units. E.g. the occurrence of a member /s/ generally after morphemes ending in voiceless consonants, and a member /z/ after voiced, is true of the morphemes {s} ‘plural’, {‘s} ‘possessive’, {‘s} ‘is’, though these units differ in their other members. If we generalize the alternation to one of voiceless-voiced members (not just /s-z/), we find it also in {ed} ‘past’. However, there are other alternations of members in which {s} ‘plural’ differs from {‘s} ‘possessive’: e.g. 'children', 'child’s'.
- Harris (1951), a pag.220

In terms of morphophonemes, we can notice if two alternations have some morphophoneme in common (i.e. are identical during part of their length), even if the remainder of the alternation differs. As a result, some alternations which differ from any other one, may be found to be sums of other known alternations.
- Harris (1951), a pag.227

When the difference between two members of a unit is described as the sum of two alternations, i.e. as the operation of two independent morphophonems in that morpheme, it is necessary to check whether the two alternations can be summed in any order, or whether one must be applied first.
- Harris (1951), a pag.237

When an alternation appears in very few morphemes, it depends upon convenience and upon our purposes whether we indicate it by a morphophoneme or by a list of members alternating in a morpheme. E.g. we could say that {'have'} is morphophonemically /hæv/, where /v/ is zero before {'ed'}, {'s'} ‘3rd person’, and /v/ otherwise: 'I have', 'I had'. Alternatively, we can say that there is a morpheme {'have'} with members /hæ/ before {'ed'}, {'s'}, and /hæv/ otherwise.
- Harris (1951), a pag.239