Increasingly, constitutional theorists are turning attention away from the modalities of constitutional interpretation (text, history, structure, etc.) and toward judicial outputs that, while featuring in constitutional adjudication, are something other than a court's determination of what the Constitution means. We might say that theorists are focusing less on constitutional meaning, more on constitutional doctrine. Despite this happy shift in emphasis, our collective understanding of the conceptual structure of constitutional doctrine remains woefully underdeveloped. For many, doctrine remains a conceptually undifferentiated mass of principles, reasons, tests, and frameworks. This is unfortunate, for no body of knowledge can long advance without self-critical classification. It is time, accordingly, to develop a functional taxonomy of constitutional doctrine. This Article takes a first and partial stab at such a taxonomy by distinguishing two components of judge-announced constitutional doctrine: statements of what the Court takes the Constitution to mean and instructions directing judges how to determine whether that meaning is complied with. Coining terms, I call the first type of doctrine a constitutional operative proposition, and the second type a constitutional decision rule. Drawing from such important recent Supreme Court decisions as Board of Trustees of the University of Alabama v. Garrett and Dickerson v. United States, this Article contends that vastly many constitutional doctrines are better understood not as judicial interpretations of the Constitution (operative propositions) but, rather, as instructions regarding how to decide whether the operative propositions are satisfied (decision rules). And it argues that recognizing the difference is likely to have broad consequences. For example, courts will better understand their own doctrines - better enabling them to sensibly revise and refine them - if they appreciate the respects in which a given doctrine communicates a decision rule rather than an operative proposition. Perhaps, say, operative propositions deserve greater stare decisis weight than do decision rules. Furthermore, this taxonomic distinction bears upon Congress's role in constitutional law-making. Although scholars frequently debate how much deference courts should accord Congress's constitutional interpretations, that is an infelicitous formulation of the issue. As Richard Fallon has recently taught, the truer, broader question concerns what role Congress should have in constitutional implementation. And judge-made constitutional decision rules may be congressionally defeasible where judicial operative propositions are not. Discrete payoffs from the operative proposition/decision rule distinction are valuable. But to focus narrowly on them risks missing the forest for the trees. Fundamentally, this Article offers an explicit (though partial) conceptualization of the logical structure of constitutional law - a conceptualization bearing a family resemblance to Monaghan's work on constitutional common law, Sager's exploration of underenforced constitutional norms, Strauss's defense of prophylactic rules, and Fallon's focus on constitutional implementation, yet reducible to none of them. This novel conceptualization makes better sense of much of contemporary constitutional scholarship and of many of the Supreme Court's most significant decisions. No doubt considerable distance toward a complete and precise taxonomy remains. But even incremental advances in detailing the conceptual map of constitutional adjudication can purchase large improvements in our ability to negotiate the terrain.