The morphemes are grouped into morpheme classes, or classes of morphemes-in-environments, such that the distribution of one member of a class is similar to the distribution of any other member of that class […]. These morpheme classes and any sequences of morpheme classes which are substitutable for them within the utterance […] are now grouped into larger classes (called position or resultant classes) in such a way that all the morpheme sequences (including sequences of one morpheme) in a position class substitute freely for each other in those positions in the utterance within which that class occurs. All subsidiary restrictions upon occurrence, by virtue of which particular members of one class or sub-class occur only with particular members out of another, are stated in a series of equations. The final result classes for the corpus, i.e. the most inclusive position classes, serve as the elements for a compact statement of the structure of utterances.
- Harris (1951), a pag.363