This article builds a framework drawn from historical institutionalism to analyse changes to the territorial composition of federal systems arising from the creation or admission of new sub-units. Despite the sustained interest among political scientists in the effects of federal design on ethnic conflict, economic development and prospects for democratic stability, there has been little sustained attempt to explain when and why territorial maps change over time. A historical institutionalist framework draws attention to the ways in which constellations of internal borders are underpinned by – and reproduce – patterns of power. The framework explains territorial change by studying the multiple layers that structure political life in federal settings and which through their interactions produce change. The article proceeds to explain territorial change in two countries with contrasting federal origins: India and the United States. In so doing, it questions the tendency within comparative politics to treat both countries as places of exception.