 This section presents a discussion of relations between individuals of distinct types, introducing a notation for types, \(t\pmsq x\) for the type to which x belongs. This section is little used in Volume I. The special consequences for this notion when dealing with relative types of cardinal numbers is the topic of the Preface to Volume II, which was added after the first volume was already in print. The delay due to working out these details partially explains the three year gap between the publication of Volume I in 1910, and the remaining volumes II and III in 1913. Section ∗65 (On the Typical Definition of Ambiguous Symbols), is a discussion of typical ambiguity, the ambiguity of variables with respect to type.