The design of subnational administrative units is fundamental to the functioning of states and their political topography. Yet, we lack theoretical and empirical knowledge on the determinants of the partitioning of state territories into subnational governance units. I argue that aligning administrative borders with ethnic geography increases short term governance efficiency. Yet, because such packing of units stabilizes ethnic identities and institutions, some governments may instead design heterogeneous units and crack groups to centralize power. I analyze these arguments by studying unit-design in Sub-Sahara Africa, contrasting different forms of colonial rule with new data on administrative and precolonial ethnic geographies derived from historical maps and toponyms. Modeling administrative borders with a probabilistic spatial partition model, preliminary results show evidence of packing rather than cracking: Ethnic boundaries are strongly and positively associated with administrative borders. These effects are most pronounced under more indirect British rule. In turn, no systematic effects are observed under more centralizing French rule. With novel theory, data, and methods, this paper contributes to our understanding of the ethnic roots of administrative geographies.