The imperfections of study outside of Persia, again, made it essential to apply to the Gaonim for authoritative expositions of difficult passages in the Bible and the Talmud.

To all such enquiries the Gaonim sent responses in the form of letters, sometimes addressed to individual correspondents, sometimes to communities or groups of communities.

These Letters and other compilations containing Halachic (or practical) decisions were afterwards collected into treatises, such as the "Great Rules" (_Halachoth Gedoloth_), originally compiled in the eighth century, but subsequently reedited.

Mostly, however, the Letters were left in loose form, and were collected in much later times.
The Letters of the Gaonim have little pretence to literary form.

They are the earliest specimens of what became a very characteristic branch of Jewish literature.