Frequently […] we find that there is a set of morphemes in a language, such that each morpheme in the set is identified by its contrast with all the others in the set. Such morphemes are often arranged in paradigms, and the various crisscrossing relationships among the morphemes of the paradigm are often called categories. Thus there are such Latin morphemes as '–us', '-um', '-ī,' '-ōs' which are defined by their membership in a paradigm, and which are considered as expressing, within the paradigm, such categories as case and number.
- Harris (1948), Pag. 87