In some cases we will find that the range of environments of one class is roughly the sum of environments of two or more other classes […]. The convenience of defining a class as a sum of other classes is particularly great when we have not a large class like 'G', but a class of one morpheme, e.g. the morpheme /tuw/. This morpheme occurs in a unique range of environments, and would therefore have to constitute a class by itself. However, it turns out that these environments are roughly equal to those of 'three', 'four' ('How much is — plus six?'), plus those of 'with', 'at' ('Don’t talk — him'.), plus those of 'also' ('I’m going —'.), plus certain unique positions ('I want — go on'.). In such cases we may decide to assign /tuw/ as member of the three recognized classes (of 'three', of 'with', and of 'also'), and as a member of a small class of its own, restricted to a few types of environment.
- Harris (1951), Pag. 257