Why are states jointly members in certain intergovernmental organizations (IGOs) but not others? Despite the proliferation of IGOs and renewed interest in this topic, we lack systematic research to answer this question. Our theory of political community explains why dyads of states are likely to be common members in particular types of IGOs. We analyze and compare functionalist and Deutschian communitarian perspectives about IGO memberships. We test our theory using newly available data on IGO mandates and institutional structure, which allows us to make specific predictions about the types of IGO to which dyads become members. We show that dyads that are economically dependent and/or democratic and enjoying enduring peace are more likely join those IGOs that possess high levels of institutional structure. Militarized interstate conflicts reduce the likelihood of states sharing membership in common IGO, but not substantially, whereas development and alliances also increase IGO memberships between states. Trade ties, however, are the most important determinant of joint membership between states in the most institutionalized IGOs, which is congruent with security communities.