English commercial law is commonly said to have developed by a process of incorporation of the law merchant. In rough form, the conventional theory is that before the seventeenth century commercial cases were not heard in the regular common law courts but in specialized mercantile tribunals associated with fairs and principal cities and towns. Cases brought in these courts were decided not by the regular judges but by the merchants themselves. The substantive law applied was not the common law but the law merchant, a specialized body of transnational customary law based on commercial practice and uncluttered by the technicalities of the common law. By the sixteenth and seventeenth centuries, however, the mercantile courts of the fairs and towns went into decline, and merchants were forced to bring their cases in the common law courts. Initially the judges of the common law courts were unfamiliar with and even hostile toward the law merchant. At most, the common law courts would treat the principles of the law merchant as customary rules that required specific proof in each case. In time, the antagonism of the common law judges was overcome, and the courts began to treat the rules of the law merchant as authentic principles of law, binding of their own force without special proof as custom. By the end of the seventeenth century, the courts began to recognize explicitly that the law merchant was part of the common law.