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Rabbeinu Gershom (961–1040), known by the title Me’or HaGolah, was a Rishon active in Metz and Mainz who stood at the head of the early Ashkenazic Torah world. He is best remembered for the binding communal enactments that bear his name (takkanot Rabbeinu Gershom), which later authorities treat with great weight. The Chochmat Adam discusses how one who violates the decrees of Rabbeinu Gershom is handled — distinguishing between an unintentional transgression, which requires no formal release, and a deliberate one after warning, for which any group of ten is empowered to release him when he repents.
There are 3 famous Cherem De’rabeinu Gershom – which were initially decreed for 500 years, but have since been incorporated as general practice. 1. Prohibition against polygamy, 2. Prohibition against opening someone else’s personal mail, 3. Ban on forced divorces.
Interestingly, Rashi was born in 1040 CE, the year that Rabbeinu Gershom died.

His standing as a foundational halachic figure is reflected in the way later codifiers engage matters connected to his era and his rulings. He is cited in the literature surrounding the laws of treifot and the inspection of animals and birds in Yoreh De’ah, as well as in discussions touching the laws of nesiat kapayim — the priestly blessing — in Orach Chaim. Through his teaching and his decrees, the lines of Ashkenazic learning that followed him drew directly upon his authority.