Al-Faḍl b. Sahl b. Zādhānfarrūkh (b. c. 154/771, d. 202/817-8) was an important ʿAbbāsid administrator ( kātib, wazīr , and amīr ). He served as mentor to ʿAbdallāh al-Maʾmūn, the son of the caliph Hārūn al-Rashīd (r. 170-93/786-809), and then as al-Maʾmūn's advisor, secretary, governor general in the east, and head of his civil and military administration (al-Maʾmūn was to reign as caliph 198-218/813-33). Because al-Faḍl's brother, al-Ḥasan b. Sahl (d. 236/850-1), worked