In some cases of classification it is not essential to select one of the members as primary in respect to the other members classified with it. E.g. in grouping complementary segments into one morpheme, we may regard one member as representing the morpheme, and call the other members positional variants of that member in stated positions.
- Harris (1951), a pag.307, n.14

A feature of a construction may also be the primary of one of its classes over the others. For example, 'X' could be considered primary and 'Y' secondary in a construction if 'X' occurred in every instance of the construction while 'Y' appeared only in some instances.
- Harris (1951), a pag.328

However, it issometimes convenient to consider one of the members to be the symbol of the new class: that member is then said to be primary (or the base) while the other members are derived from it by a set of environmentally or otherwise) conditioned ‘rules’ or operations. For example, we may say that the phoneme /t/ is the member segment [t] plus various changes in various positions. Or we may say that the morphophoneme /F/ is the phoneme /f/ plus the change to voicing before {'-s'} ‘plural’. We can even say that the Semitic position class 'N' is the (void) morphemic component '3' ‘third person’ plus various residues (for ‘first person’, for ‘book’, etc.) in various of its occurrences. In all these cases, we would consider one member 'a' as primary if we can state the conditions in which the other elements 'b', 'c', replace it (are derived from it). The choice of 'a' is clearer if we can not reversibly derive 'a' from 'b' or 'c'; i.e. if we can not state the exact conditions in which 'b' is replaced by 'a'. When no member of a class can be set up as primary, it may be possible to set up a theoretical base form from which each member can be derived (cf. in morphophonemics). In all these cases, however, whether we set up a primary member, or a theoretical base form, or a new class of the old members, we have essentially the same relation: a number of elements, classified together on some basis, into a new element which represents the occurrence of each of them.
- Harris (1951), a pag.367, n.9