Ulugh Beg’s zīj, an astronomical handbook with tables and new star catalogue was the result of innovative observational and calculational methods developed by Ulugh Beg and his colleagues at the Samarqand observatory to rectify errors found in the Zīj-i Īlkhānī, which was produced at the Maragha observatory under the directorship of Naṣīr al-Dīn al-Ṭūsī (d. 672/1274). Several decades after the zīj’s completion (between 878/1473 and 879/1474), Qūshjī, a prominent contributor to the zīj’s preparation, composed a commentary on the Zīj (Sharḥ-i zīj-i Ulugh Beg) that became highly influential in both Ottoman and Persian lands. The commentary not only includes corrections and criticisms of the text, but also explains techniques used in the production of the Zīj.