An influential medieval rabbinic authority, often referred to as the Ba’al ha-Turim, “Master of the Turim (Pillars),” after his main work in halakhah, the Arba’ah Turim. He was the third son of the Rabbi Asher ben Jehiel (known as the “Rosh”), a German-born rabbi who moved to Spain. Besides his father, who was his principal teacher, Jacob quotes very often in the Turim his elder brother Jehiel; once his brother Judah, and once his uncle R. Chaim (1270–ca. 1340).