Minhag—custom מנהג
"Your fathers have already accepted it upon themselves"—and from that point a custom stops being a habit and becomes an obligation. On custom stands half the Shulchan Aruch: the half the Rema wrote.
Custom is an oral and local thing. It lives by having been seen: how it is done here, how they sing here, what is not done here. The question is always the same: does it bind—and whom.
Where the obligation comes from. The Gemara (Pesachim 50b) tells of the people of Beishan, who did not travel on business from Tyre to Sidon on the eve of the Sabbath. Their children asked Rabbi Yochanan whether they too were bound. The answer was: 'Your fathers have already accepted it upon themselves'—with the verse 'hear, my son, the instruction of your father' (Proverbs 1:8). That is: what the fathers accepted binds the children.
This was seriously disputed. At Rosh Hashanah 18b the four fasts are discussed, whose obligation depends on circumstances; Nachmanides holds that once the people have accepted it a custom binds of itself, while the Rashba objects that general acceptance is not enough for a strict obligation.
Why this is no small matter. Because custom is what distinguishes communities from one another, while their law is common. Ashkenaz and Sepharad differ not in what is forbidden but in how it is done. And when Rabbi Joseph Karo issued the Shulchan Aruch according to Sephardi usage, the Ashkenazim needed the Rema and his Mapah—glosses bringing in their custom. Since then the book is one, and it is lived by in two ways.
And hence the price of writing it down. While a community stays put, custom passes on of itself. When communities are driven from place to place it must be written, or it is lost. So in the fifteenth century the Sefer Maharil appeared—customs recorded by a student after his teacher—and from there nearly all of it passed to the Rema. There is a separate event on this here.
On the timeline
Related
Sources
Chazal: Mishnah, Talmud, midrash, Zohar
- Pesachim 50b — the people of Beishan and Rabbi Yochanan's answer: 'your fathers have already accepted it upon themselves'
- Rosh Hashanah 18b — the dispute of Nachmanides and the Rashba on whether a custom accepted by the people binds of itself
Acharonim and later rabbinic literature
- The Rema's Mapah on the Shulchan Aruch — the bringing of Ashkenazi custom into a Sephardi code
Basis: documents and evidence.