This book tells the story of the formation of a new field of knowledge. It shows how various authors combined their knowledge, experience, and critical thought to write lawbooks that made various disparate customs into a field of knowledge known as customary law. These authors wrote texts, known as the coutumiers in the French legal tradition, in thirteenth-century northern France. ‘Customary law’ typically refers to a type of rule made in practice, and in the courts, by the community, which can include ‘the people’ in some form, lords and kings, or lawyers and judges. Customs concerning specific rules of property, succession, and other subjects certainly emerged out of this oral practice. Coutumier authors, however, successfully crafted customary law into an expository genre of writing, one that took ideas of custom from practical experience and different forms of book learning and transformed them into bodies of customary law.