To decide many cases, courts need to characterize some of the legal rules involved, placing each one in a specific doctrinal category to identify the rule's effect on the litigation. The consequences of characterization decisions can be profound, but the grounds for making and justifying them are often left unstated. This Article offers the first systematic comparison of two important types of legal characterization: the distinction between substantive and procedural rules or statutes, a

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... tion federal courts make in several contexts; and the distinction between jurisdictional and nonjurisdictional rules, especially those relating to litigation filing requirements. The Article explains the reasons for the differences between the doctrines governing each type of characterization by contextualizing each as an example of the same activity: the identification of the "genre," or kind, to which particular legal texts belong. Showing that decisions in both areas do in fact involve genre classification, the Article explains how it follows that legal characterization is an aspect of legal interpretation, although courts have seldom recognized as much. This analysis further suggests new lines of development for both Erie doctrine and jurisdictional characterization. Judges making Erie decisions should characterize both the state and federal laws at issue according to their sources, as well as under the substantive-procedural rubric, and should recognize that the question of conflict, if reached, is akin to other questions of federal preemption. Judges making jurisdictional-characterization decisions should extend the existing doctrinal framework to take into account other consequences of characterization and to allow the analogous handling of federal rules. The U.S. Supreme Court already has most of the resources it needs to move down these paths. Still,