This chapter provides a history of the idea of custom both in definitions and as presented in texts about lay legal life before the coutumiers. The first part of the chapter examines the struggle to define custom from late antiquity to the time of the coutumiers. It shows that thinking about and defining custom in its legal capacity was no obvious thing: while there where common definitional elements, the quest to define custom was marked by debate and a lack of consensus that would continue into the fourteenth century.