Rabbi Yoel ha-Levi of Bonn רבי יואל הלוי מבונא

Bonn on the Rhine was one of the towns where the community gathered itself again after the massacres of the First Crusade. Its yeshiva was kept by Rabbi Yoel ha-Levi, a grandson of the Raavan through his mother and a man of the same school: he drew the law from the sugya and checked the custom against the households where he found it alive.

He left almost no books of his own—rather, they did not survive. But his words were preserved more securely than books: his son, Rabbi Eliezer ben Yoel ha-Levi, the Raavya, wrote them down, and in the Aviezri his father's rulings are cited by the dozen. Through the Raavya they entered the Or Zarua, from there the Mordechai, and from there the Ashkenazi rite still prayed today.

That is how transmission works in this school: the teacher speaks, the student writes, and two links later what was said in the Bonn beit midrash becomes law for communities the speaker never heard of.