Custom occupied a considerable part of the exposition of Islamic law concerning exchange. It was used by the Islamic scholars as a reference and guide for the licitness and acceptability of commercial practices. It was also a recognition and a reflection of the prominence of personal relations as a concomitant element in economic and commercial relations. This, in turn, had implications for the size of enterprises, so that these were generally of modest size and had a highly fragmented division of labour. For Muslim economists and historians. Analytical. Based on primary sources. Documented.