A class whose members occur in the same external environment, e.g. all verbs which may occur befor the suffix '-ing', constitutes an external distribution class. However, once the suffix '-ing' has been added, we describe such a series as an internal distribution class, since they all have the same morpheme in common. Such a group, however, constitutes another external distribution class in that its members may all follow the verb 'is': 'is working', 'is walking', 'is playing', 'is dancing'.
- Nida (1949), a pag.91

An 'external distribution class' consists of a set of forms which occur in the same environment. Every internal distribution class (except, of course, the discourse) has an external distribution.
- Nida (1949), a pag.110

The major external distribution class membership, e.g. noun, adjective, verb, adverb, pronoun, must be distinguished from the limited external distribution class membership, since by occurrence with the same determiner any determined series constitutes a limited external distribution class.
- Nida (1949), a pag.120