The term "region" is used in many different ways. 1 For example, "region" is occasionally used at a macrolevel to refer to groups of nations. In this chapter we refer to a more limited, intranational pattern of regular social relationships. For our purposes, "region" means the arena of potential "day-to-day" social interaction, the set of normally available possible social interactions. The notions of "travel to work area" and "standard metropolitan statistical area" are spatial reflections of regional structures in the sense intended here.