However, it is sometimes 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

Since each element is identified relatively to the other elements at its level, and in terms of particular elements at a lower level, our elements are merely symbols of particular conjunctions of relations: particular privileges of occurrence and particular relations to all other elements. It is therefore possible to consider the symbols as representing not the particular observable elements which occupy an environment but rather the environment itself, and its relation to other environments occupied by the element which occupies it. We may therefore speak of interenvironment relations, or of occupyings of positions, as being our fundamental elements.
- Harris (1951), a pag.370

When we get a class of segments which are free variants of each other, we use one symbol which indicates any member of that class: we write just [kʰ] both in 'can’t' and in 'cameras'. Any differences we may have noticed among the equivalent segments are henceforth disregarded. This reduces considerably the number of different symbols (or of differentiated segments) in our record of utterances.
- Harris (1951), a pag.33