during the Middle Ages, the body of customary rules and principles relating to merchants and mercantile transactions and adopted by traders themselves for the purpose of regulating their dealings. Initially, it was administered for the most part in special quasi-judicial courts, such as those of the guilds in Italy and, later, regularly constituted piepoudre courts in England (see piepoudre court).

The law merchant was developed in the early 11th century in order to protect foreign merchants not under the jurisdiction and protection of the local law. Foreign traders often were subject to confiscations and other types of harassment if one of their countrymen had defaulted in a business transaction. A kind of law was also needed by which the traders themselves could negotiate contracts, partnerships, trademarks, and various aspects of buying and selling. The law merchant gradually spread as the traders went from place to place. Their courts, set up by the merchants themselves at trade fairs or in cities, administered a law that was uniform throughout Europe, regardless of differences in national laws and languages. It was based primarily on Roman law, although there were some Germanic influences; it formed the basis for modern commercial law.