The language used by the Gaonim was at first Hebrew and Aramaic, and the latter remained the official speech of the Gaonate.  In course of time, Arabic replaced the Aramean dialect, and became the lingua franca of the Jews.

The formal works of the Gaonim, with certain obvious exceptions, were not, however, the writings by which they left their mark on their age.  The most original and important of the Gaonic writings were their “Letters,” or “Answers” (Teshuboth).  The Gaonim, as heads of the school in the Babylonian cities Sura and Pumbeditha, enjoyed far more than local authority.  The Jews of Persia were practically independent of external control.  Their official heads were the Exilarchs, who reigned over the Jews as viceroys of the caliphs.  The Gaonim were the religious heads of an emancipated community.  The Exilarchs possessed a princely revenue, which they devoted in part to the schools over which the Gaonim presided.  This position of authority, added to the world-wide repute of the two schools, gave the Gaonim an influence which extended beyond their own neighborhood.  From all parts of the Jewish world their guidance was sought and their opinions solicited on a vast variety of subjects, mainly, but not exclusively, religious and literary.  Amid the growing complications of ritual law, a desire was felt for terse prescriptions, clear-cut decisions, and rules of conduct.  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.  “Questions and Answers” (Shaaloth u-Teshuboth) abound in later times in all Jewish circles, and there is no real parallel to them in any other literature.  More will be said later on as to these curious works.  So far as the Gaonic period is concerned, the characteristics of these thousands of letters are lucidity of thought and terseness of expression.  The Gaonim never waste a word.  They are rarely over-bearing in manner, but mostly use a tone which is persuasive rather than disciplinary.  The Gaonim were, in this real sense, therefore, princes of letter-writing.  Moreover, though their Letters deal almost entirely with contemporary affairs, they now constitute as fresh and vivid reading as when first penned.  Subjected to the severe test of time, the Letters of the Gaonim emerge triumphant.