Article 38, paragraph 1(b), of the Statute of the International Court of Justice (ICJ) is universally considered to be an authentic definition of custom as a principal source of international law, not least by the International Law Commission (ILC) in its recently completed work on the identification of international customary law. At the same time, though, the formula has constantly met with severe strictures concerning its very formulation. Given the text as it stands, this paradoxical dissonance cannot be satisfactorily resolved, but it can at least be tentatively explained by retracing the genesis of the clause a century ago, at the price, however, of a complete reappraisal of the whole of Article 38. Notoriously, Article 38 originated in a proposal of Baron Descamps, the president of the Advisory Committee of Jurists in charge of devising the Statute of the Permanent Court of International Justice (PCIJ). Yet the ambivalent but crucial role of the Descamps Proposal in drafting the article has not hitherto been realized. In fact, owing to the debates it aroused and to its misapprehension as a draft article instead of a merely exploratory basis of discussion, it has directly led to the shortcomings of Article 38 and especially of its clause on customary law.