Head of the Yeshiva in Sura, d. 875. Student of Rav Natronai Gaon Author of about 120 Response,  שערי צדק. His commentary on, and outline of the order of the prayers – Siddur – was basically the first written version of its kind. It was heavily influential on the Siddurim of Rav Sa’adia Gaon (d. 942) and the Rambam and forms the basis of the Ashkenazi nusach to this day.