The conceptual categories that we have seen to be organized into typological markedness and hierarchy patterns do not occur in isolation. Any given noun phrase has values for number, case and animacy/gender, for example, and any given verb has values for tense, aspect, modality and so on. In other words, conceptual categories always occur in combination in utterances. Hence, it is reasonable to examine the possibility of grammatical interactions between conceptual categories, and to seek typological patterns in those interactions. This chapter will explore some of these patterns.

In fact, we have already seen one instance of interaction of categories. Behavioral potential was described in §4.1.2 as the potential for a category value to inflect for values of other categories, or to have a distribution in a wider range of syntactic contexts. It was assumed that the typological markedness ranking of values of a category would be the same no matter what potential is examined. But in fact the rankings can and do vary depending on what other category is being used for measuring behavioral potential. These are illustrated in §6.1

This is not the only type of interaction between categories, however. The second type is an interaction of values in two cross-cutting categories. Here, a value in one category is typologically unmarked only if it co-occurs with a particular value of another category. The values on the two categories form a cluster or typological prototype.