In Chapter 3 we made the point that a lexicon is a congeries of words, a classification is a set of classes, but a typology is a system of types. What we mean by this is that a typology has systemic or structural features that are not entirely dictated by the nature of the constituent types, just as a language has structural features that are not dictated by its lexicon (cf. Waugh 1976: 20). In this chapter we will consider first what are the structural features of a typology, and then how they come about and what (if any) purpose they serve. The features that we will specifically discuss are those of boundedness and comprehensiveness of the typology itself, and the mutual exclusiveness, consistency of definition, equivalence, equidistance, and independence of the constituent types.

Basic structural features

Boundedness. In any classification that is to be used for sorting entities (that is to say, a typology), it must be clear at the outset what is and is not to be sorted. Hence typologies (unlike some other classifications) have quite rigidly specified and inflexible boundaries. Moreover, because sorted material is often subjected to statistical manipulations, the outer boundaries of the system cannot be adjusted to include new material which was formerly excluded, without invalidating any statistical operations that were previously performed.

Typological boundaries may be defined on the basis of either internal or external criteria, or both (see Chapter 14).