Customary international law is one of the two main sources of international law. Yet there remains considerable uncertainty about the process through which rules of custom emerge or subsist - the â € meta-law of customâ €™, which is now under consideration within the un International Law Commission (ilc). This article does not rehearse arguments about these uncertainties nor indeed engage with the current work of the ilc. Instead, it focuses on areas of certainty, viz. aspects of the law of meta-custom that are generally agreed and on which the ilc can draw. It argues that this certainty is the product of decades of jurisprudence, first of the Permanent Court and then of the International Court of Justice. In highlighting four crucial contributions and situating them in the debate about judicial law-making, this article seeks to raise awareness for the World Court's (often unacknowledged) role in shaping the meta-law of custom.