Custom was fundamental to medieval legal practice. Whether in a property dispute or a trial for murder, the aggrieved and accused would go to lay court where cases were resolved according to custom. What custom meant, however, went through a radical shift in the medieval period. Between the twelfth and thirteenth centuries, custom went from being a largely oral and performed practice to one that was also conceptualized in writing. Based on French lawbooks known as coutumiers, Ada Maria Kuskowski traces the repercussions this transformation – in the form of custom from unwritten to written and in the language of law from elite Latin to common vernacular – had on the cultural world of law. Vernacular Law offers a new understanding of the formation of a new field of knowledge: authors combined ideas, experience and critical thought to write lawbooks that made disparate customs into the field known as customary law.