Often, when we speak about authority of law, we mean two different things. On the one hand, authority of law relates the status of law as a domain of practical reasons. This means that law's authority is an impact it has on people's practical deliberations. On the other hand, authority of law manifests in certain formal institutions that create, apply, enforce, etc. legal norms. Here, authority of law is not so much about its practical impact, but about public authorities that claim normative powers. The common jurisprudential account of authority typically entails that these two meanings of authority are inseparable; there cannot be one without the other. Law's authority as a practical guide depends on there being formal institutions which make legal claims. In this contribution, I show that international law allows to reveal limitations of this approach. In the context of customary international law, which is the primary focus of the article, the gap between the two meanings of authority becomes strikingly visible. Customary international law claims and has the authority without necessarily having a link to formal institutions. I suggest that there are two types of authority of law: mediated and unmediated. Analytical jurisprudence typically accounts only for the first one, but remains silent about the second. However, if we accept that authority can in principle be unmediated, this entails that formal institutions are not necessary ingredients in the normative structure of authority. We should therefore shift the focus in jurisprudential explanations of authority from institutions to norms and their pre-emptive capacities. I suggest that the success with which norms manage to pre-empt first-order reasons is the primary manifestation of the authority of law. I claim that it is not necessary for a norm to be an outcome of delegated practical deliberations in order to be a pre-emptive normative standard. Both mediated and unmediated structures of authority allow to realise the pre-emptive function of norms, albeit by different means.