Unwritten rules (synonyms: 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."