Whether a limitation is jurisdictional or not is an important but often obscure question. In an article published in Northwestern University Law Review, I proposed a framework for courts to resolve the issue in a principled way, but I left open the next logical question: what does it mean if a rule is characterized as nonjurisdictional? Jurisdictional rules generally have a clearly defined set of traits: they are not subject to equitable exceptions, consent, waiver, or forfeiture; they can be raised at any time; and they can be raised by any party or the court sua sponte. This jurisdictional rigidity has led courts and commentators to overlook the fact that nonjurisdictional rules need not be the mirror inverse but may instead have attributes commonly associated with jurisdictionality. A nonjurisdictional rule might, for example, be "mandatory," meaning that it is subject to waiver or forfeiture, but if properly raised by the party for whose benefit it lies, it has the jurisdictional-like attribute of being immune to equitable exceptions. This Article is the first to take a hard look at nonjurisdictional rules and, particularly, "mandatory" rules. It first argues that they have an important institutional role to play in our procedural system. It then shows that, in practice, mandatory but nonjurisdictional characterizations may help explain a number of perplexing doctrines. As an example, the Article demonstrates how such a characterization can help reconcile the convoluted doctrine of state sovereign immunity. Ultimately, the Article suggests that a greater appreciation for mandatory rules both can benefit the procedural system and can broaden our view of what salutary roles nonjurisdictional rules can play.