Rabbeinu Gershom רבינו גרשום מאור הגולה
leader of Ashkenazi Jewry, author of the decrees on marriage and the privacy of letters
Rabbeinu Gershom lived in Mainz on the Rhine, where the first large Ashkenazi communities were taking shape. He led an academy that drew students from Germany and France, and Rashi was taught by his students' students.
His lasting legacy is the «takkanot of Rabbeinu Gershom», decrees adopted by an assembly of communities and binding on all Ashkenazi Jewry. First: the ban on polygamy — by the letter of the Torah a man could have several wives, and Rabbeinu Gershom ended it, so that the Jewish family in Europe became monogamous. Second: a man may not divorce his wife against her will. Third: one may not read another person's letter — the oldest rule of postal privacy in history.
A fourth, less known, forbids reproaching a Jew who was baptised under threat of death and has returned. It was written by a man whose own son, tradition says, was forcibly baptised and died before returning; Rabbeinu Gershom mourned him as a Jew.
He also wrote Talmudic commentaries, printed where Rashi's are missing, and worked to establish an accurate Talmudic text.