Must a federal court have obtained the power to bind a party before her citizenship becomes relevant to diversity jurisdiction? For a long time conventional wisdom has assumed the answer is “no”: Congress can authorize diversity jurisdiction based on the citizenship of persons who, although currently beyond the court’s power to bind, might later join the suit. Congress, in turn, has acted on this assumption. Key provisions of the most ambitious, and controversial, expansion of diversity jurisdiction in the last decade, the 2005 Class Action Fairness Act (CAFA), hinge diversity jurisdiction on the citizenship of persons conventionally believed beyond a court’s power to bind—i.e., proposed class members in an uncertified class. Based on an examination of the original semantic meaning of the Diversity Clause, this Article argues that the conventional wisdom is wrong: Diversity jurisdiction is limited to suits in which citizens of different states are brought within a court’s power to bind their interests. In the process, the Article sheds new light on the original meaning of an Article III “controversy”—in particular, on whom an Article III “controversy” subsists “between.” The Article ends by exploring the ramifications of the Clause’s original meaning. First, recovering that meaning reveals how Article III and due process norms combine to protect states from jurisdictional encroachment by federal courts. Second, the original meaning of the Clause provides a powerful textual basis for involving Congress in important decisions about the outer reach of federal principles of nonparty preclusion. The Article illustrates these points, and shows how they are related, by applying the original meaning of the Diversity Clause to resolve questions about the constitutionality of the Class Action Fairness Act.