This chapter argues that the emergence of what we recognize as the modern international system develops through evolving practices of diplomacy. IR literature has increasingly paid attention to the early modern development of diplomacy to understand the origins of the system. This chapter offers a distinct interpretation of the importance of diplomacy from the perspective of the closure thesis. In contrast to the typical account of diplomacy as mediating the political fractures that resulted from the breakdown of Christendom, it argues that the adoption and diffusion of specific diplomatic practices, such as the permanent resident ambassador, facilitated closure and boundary-drawing by narrowing the types of actors invested with rights of political representation. Diplomatic practices emerged in part as a means of producing common goods and securing privileged access to those goods for some political actors, while facilitating the political exclusion and subordination of others. The chapter concludes with a discussion of the argument’s contemporary relevance. In reflecting on an historical age in which the sovereign state was not yet the only legitimate political agent, the contemporary question is whether today’s legal and representational rights at the global level can or should be emancipated from the state.