Law is often taken as normative – power-conferring and duty-imposing – by virtue of conventions. Despite their foundational place in law, these conventions are often explained away, rather than carefully theorized. Either conventional practices are reduced to bare facts of instrumental rationality and incentives; or to robust already-normative facts about public morality, human rights, democratic legitimacy, etc. I argue against both of these approaches. We need to explain conventions in a way that distinguishes between what is legal and what is morally legitimate, disqualifying the latter approach. We also have to distinguish between brute force and law, disqualifying the former. I propose a way of maintaining both distinctions: taking conventions as irreducibly social (rather than moral) and intersubjective (rather than atomistically instrumental). Such an approach might allow for a more robust theory of the foundational rules of legal systems, and richer accounts of non-state law.