Some of the features of selection, the restrictions on particular morphemes which occur only with particular other morphemes, are indicated in these [construction] formulae, or derivable from them. Some selection, such as that between 'I' and 'am' as against 'he' and 'is', is included in the list of variant forms of the morphemes. Selections of concord are listed as special domains of the morpheme in question […].
- Harris (1946), a pag.180

The great bulk of selection features, especially those that distinguish between individual morphemes, cannot be expressed except by very unwieldy formulae. Although it may be of theoretical interest to know that two-dimensional diagrams of such detailed selections are conceivable, in practice this information can only be given in lists and statements appended to the formulae […]. This is true also of such relations among morphemes as the families of mutually replacing English suffixes, e.g. '-id', '-or' in 'squalid: squalor, candid: candor', etc.
- Harris (1946), a pag.182