Five years ago, the U.S. Supreme Court was asked a narrow question: Can a federal court exercise so-called Class Action Fairness Act (CAFA) jurisdiction—the form of jurisdiction in 28 U.S.C. § 1332(d)—over a putative class action that, if brought instead in the state court, contains claims that could not be brought on a class action basis? The Court’s answer, because of federalism and the Rules Enabling Act, was yes. A claim that could not be brought on a class action basis in state court, because state law prohibits those claims from being brought on that basis, could be brought on a class action basis in federal court because Rule 23, the class action rule, is a procedural rule and, as a procedural rule, is valid in all federal courts. (Briefly stated, under the Erie doctrine, federal courts may apply federal procedural law but must apply state substantive law.)