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Analyses of residential mobility are usually conditioned on a system of geography in which territory is divided into discrete units. Types of movement are defined in terms of these units, the most important distinction being that between local mobility and migration. Here we examine explicitly the implications of the choice of the migration-defining boundary in the U.S. over the 1940–1980 period. We demonstrate how boundary choice influences the extent and character of selectivity of the mobile population by using demographic and social characteristics. It appears that over time the state line may be replacing the county line in distinguishing kinds of migrants. Further, our results point to a growing fraction of footloose migrants, not tied to local territory, identified by their migration history rather than demographic characteristics.