The earliest existing codification of the prayerbook is the Siddur (order) drawn up by Amram Gaon of Sura about 850.
Half a century later the famous Gaon Seadiah, also of Sura, issued his Siddur, in which the rubrical matter is in Arabic. Besides the Siddur, or order for Sabbaths and general use, there is the Mahzor (cycle) for festivals and fasts.
He was the first to arrange a complete liturgy for the synagogue, and his Prayer-Book (Siddur Rab Amram) was the foundation of most of the extant rites in use among the Jews.
The Siddur was published in Warsaw in two parts (1865).
Besides minor works, such as a recension of the Prayer-Book (Siddur), the Pardes and ha-Orah, Rashi wrote two great commentaries on which his fame securely rests.