Abstract: Recent studies of commercial conflict resolution have emphasized the role of informal norms and extralegal incentives as compared to the formal legal system. Yet the merchants who frequented medieval English fairs, whose example has been invoked as a precedent for modern dispute resolution, may not have fit this model. These merchants frequently litigated before the courts of the fairs, local tribunals of general jurisdiction that retained formal procedures and traditional methods of proof. Why did these traders rely on existing authorities rather than their own private institutions? And why did they appear before local tribunals, rather than alternative fora such as the English royal courts? This essay examines the records of the fair court of St. Ives, one of England’s largest and best-documented fairs in the late thirteenth and early fourteenth centuries. It argues that the fair court managed to attract litigants in the face of jurisdictional competition through an effective alignment of legal and extralegal incentives. The court offered not only reputational sanctions, but also the coercive process necessary to govern a heterogeneous trading community. Although it lacked the reach and authority of a royal court, it offered merchants greater speed and flexibility in the application of specific customs, relying on community knowledge rather than official fact-gathering. The fair court of St. Ives provides an illuminating example of the interaction of law and society, demonstrating how fragile legal systems can succeed by making use of, and coordinating with, extralegal norms and incentives to accomplish official ends.