unwritten
adj.adj. describing a rule or agreement that everyone knows and follows, even though it is not recorded in an official document.
adj. existing by custom or common understanding rather than being formally recorded in writing. Often used to describe social norms or traditional legal principles.
There is an unwritten rule about who sits where at lunch.
In many offices, there is an unwritten agreement that the person who finishes the coffee must brew the next pot.
The stability of the parliamentary system relies heavily on unwritten conventions that dictate how power is transferred between parties after an election.
From Middle English unwriten, from Old English unwriten, unġewriten (“unwritten”), equivalent to un- + written.
Typically used attributively before a noun like 'rule', 'law', or 'agreement'.