She'elot u-Teshuvot—the responsa שאלות ותשובות

It began with geography. After the Muslim conquest the Jews found themselves in one state from Spain to Persia and scattered across it; a community at Kairouan had no one to ask, and wrote to Baghdad. The geonim answered, the answers were copied and kept. So the genre took shape.

After that it did not stop for a thousand years, and has not stopped yet. There are responsa of the geonim, of the Rif, of Maimonides, of the Rashba, of the Maharam of Rothenburg, of the Radbaz, of Isserlein, of Bruna, of Ergas, of the Chatam Sofer—and onward without a break.

How this differs from a code. A code says how one acts in general. A responsum answers a case: this is what happened, this is what is asked, this is the answer. So in the responsa one sees what is nowhere else: prices, crafts, contracts, neighbours' quarrels, the position of women, dealings with gentiles, flight from a pogrom, the ransom of captives, the questions of anusim about what they are now to do. Historians read them as the chief source on Jewish daily life—and not because the author meant to leave testimony but because he did not: he was deciding a case.

How it is built. The question sets out the circumstances; the answer works them through the Talmud and the predecessors and ends in a ruling. The asker is not bound to obey just anyone—he chooses whom to write to, and the answerer's authority rests on recognition. By this Jewish legal life went on without a central power: by letters.

In this reference work responsa stand as the source for dozens of articles—and almost always where one needs to know not what was ruled but what was.