The political convulsions of the past decade have fueled acute interest in constitutional norms or “conventions.” Despite intense scholarly attention, existing accounts are incomplete and do not answer at least one or more of three major questions: (1) What must all constitutional norms do? (2) What makes them conventional? (3) And why are they constitutional?

This Article advances an original theory of constitutional norms that answers these questions. First, it defines them and explains their general character: they are normative, contingent, and arbitrary practices that implement constitutional text and principle. Most scholars have foregone examining how norms are conventional or have relegated them to coordinating behavior, like rules requiring drivers to stick to one side of the road. By contrast, this Article argues that constitutional norms are constitutive conventions, which concretize values into practices; they are akin to conventions of etiquette that concretize concepts like “politeness.” Constitutional norms implement abstract principles, like the separation of powers, or indeterminate text, such as “advice and consent,” into specific behavior and action.

By understanding constitutional norms as constitutive conventions, this Article explains norms’ salient features, basic functions, and relationship to the Constitution. Norms are normative because they command respect and allegiance; they are contingent because they depend on political, social, and intellectual conditions to emerge and endure; they are arbitrary because they represent one of many possible ways of realizing constitutional text and principle; and they are constitutional because the values they implement arise from the Constitution itself. This Article animates its theory through case studies of three constitutional norms: blue slips, the norm against court-packing, and executive noninterference in law enforcement. It concludes by questioning the use of historical practice in constitutional interpretation. It suggests that when scholars and judges draw on norms that are intrinsically contingent and arbitrary, they embed unstated normative assumptions about the past and how it should constrain the future.