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unspoken rule

n. A rule that is understood but does not exist in written form.

Wikipedia
Unspoken rule

Unspoken rules are behavioral constraints imposed in organizations or societies that are not voiced or written down. They usually exist in unspoken and unwritten format because they form a part of the logical argument or course of action implied by tacit assumptions. Examples involving unspoken rules include unwritten and unofficial organizational hierarchies, organizational culture, and acceptable behavioral norms governing interactions between organizational members.

For example, the captain of a ship is always expected to be the last to evacuate it in a disaster. Or, as Vince Waldron wrote, "A pet, once named, instantly becomes an inseparable member of the family."

Usage examples of "unspoken rule".

One unspoken rule regarding The Fort was observed by all: The Fort did not exist.

There was also what amounted to an unspoken rule against talking about it at all, among the older generation.

There's an unspoken rule that if one court banishes someone from faerie, the other court respects the punishment.

There were attempts at conversation governed by some unspoken rule that the hanging itself should not be discussed.

She was too high to think of objecting, though it broke her unspoken rule of staying more than two nights running.