The treatment of customary practices has been widely debated in many areas of the law, but there has been virtually no discussion of how custom is and should be treated in the context of intellectual property (IP). Nevertheless, customs have a profound impact on both de facto and de jure IP law. The unarticulated incorporation of custom threatens to swallow up IP law, and replace it with industry-led IP regimes that give the public and other creators more limited rights to access and use intellectual property than were envisioned by the Constitution and Congress. This article presents a powerful critique of the incorporation of customs in IP law and provides a theoretical framework for future analysis of private ordering regimes in IP. The article also provides additional support to those who have criticized the incorporation of custom in other fields.