Minhag Hamedinah Common Commercial Practice 4 Self-Adopted Communal Practices

Q: When can self-adopted communal practices, which are not based on any regulation, become minhag hamedinah?

A: Even communal practices that are not based on any regulation, but rather people adopted these practices and act accordingly, have halachic sanction, with two provisions:

1. Most of the people there act that way, whether because most of them came from a place where everyone acted that way, or because most of the people act that way of their own accord (C.M. 218:21; Rema 331:1). Presumably, this requires a predominant majority of the people, so that it became an accepted practice in that place. Moreover, some write that so long as it is commonly known that a minority acts otherwise, the person in possession can claim that he is from the minority (Rambam, Hil. Ishus 23:12; E.H. 66:1; Nachal Yitzchak 61:5:4).
2. The practice was implemented often and is a common occurrence, so that clearly it is an accepted practice there (Rema 331:1, 163:3; Gra 163:57).