The mode, style and subject matter have changed as a function of the travels of the Jewish people and of the development of other "halakhic" literature, particularly the codes. Forumulation of responsa, or she'elot ve-t'shuvot, which literally translates to questions and answers, generally involve an individual or group asking either teachers, rabbis, or heads of yeshivot about halakhah (Jewish law) and the party responding via an exchange of letters. Responsa literature spans 1700 years and there are even responsa being developed based on questions posed today. The development of responsa literature can be divided into four periods: the Tannaitic Period, the Geonic Period, the First Rabbinic Epoch, the Second Rabbinic Epoch, the Third Rabbinic Epoch, and the Fourth Rabbinic Epoch.