Nickname for Yaqub ibn Ibrahim al-Ansari. Student of legist Abu Hanifah. Spread influence of Hanafi school through his writings and the government positions he held. Appointed judge in Baghdad, later chief justice (qadi al-qudat) under Abbasid caliph Harun al-Rashid with authority to appoint judges in the empire. Some of his opinions differ from those of Abu Hanifah, probably on the basis of traditions not available to the earlier scholar. His most famous work is Kitab al-kharaj, a treatise on taxation and fiscal problems of the state prepared for the caliph. He compiled the earliest known work of principles of Islamic jurisprudence (usul al-fiqh). Portion of work devoted to international law.