Although legal customs are nowadays a subsidiary source of law, the concept of custom is implicitly used by legal scholars to account for quasi-juridical phenomena such as the role of precedents in civil law systems, the use of foreign law by courts, the constitutional conventions, or the non-binding agreements, rules and principles of soft law. The same concepts has been also employed in legal philosophy to characterize the social practice at the foundation of law, such in the case of Hart’s theory of the rule of recognition. In this essay I first consider the problems that the traditional accounts of customary law gives rise to. Then I put forward an original account of customary law based on Gilles Deleuze’s philosophical inquiry into the concept of repetition. According to the account proposed in this essay, legal custom is an interstitial source of law which modifies the content and the conditions of application of positive law. This account also provides some new insights with regard to the contemporary debate on the rule of recognition and the normativity of law.