Institutions are norm-governed social practices, or so I propose. But what does it mean for a norm to govern a social practice? Theories that analyze institutions as equilibria equate norms with sanctions and model them as costs. The idea is that the sanctions change preferences and thereby behavior. This view fails to capture the fact that people are often motivated by social norms as such, when they regard them as legitimate. I argue that, in order for a social norm to be perceived as legitimate, agents have to acknowledge reasons for conforming to it other than the sanctions they might incur for violating it. In light of this, I defend a theory of institutions that does not only invoke equilibria, but also normative rules that are suppor...