This book presents a theory to account for why and when politics revolves around one axis of social cleavage instead of another. It does so by examining the case of Zambia, where people identify themselves either as members of one of the country's seventy-three tribes or as members of one of its four principal language groups. Drawing on a simple model of identity choice, it shows that the answer depends on whether the country is operating under single-party or multi-party rule. The book thus demonstrates how formal institutional rules determine the kinds of social cleavages that matter in politics.