In this essay prepared for the University of Pennsylvania’s conference on Intellectual Property and the Common Law, I build upon my work on custom and intellectual property. I focus here on one important facet of the subject — how longstanding common law principles should inform our understanding of custom. The common law provides a number of lessons on how to appropriately limit the consideration of custom in intellectual property law and elsewhere. The essay begins by considering the traditional role of custom in the common law. Part II then examines several of the ways that courts have incorporated custom into copyright law, particularly in the context of determining fair use. Part III explores recent efforts to use custom to ameliorate ...