Something such as a book that is unwritten has not been printed or written down. Universal have agreed to pay £2.5 million for Grisham's next, as yet unwritten, novel
said or done without having been planned or written in advance; "he made a few ad-lib remarks"
so ancient that they well might have had their unwritten origins in Aurignacian times"- J
based on custom rather than documentation; "an unwritten law"; "rites so ancient that they well might have had their unwritten origins in Aurignacian times"- J L T C Spence
An unwritten rule, law, or agreement is one that is understood and accepted by everyone, although it may not have been formally or officially established. They obey the one unwritten rule that binds them all -- no talking. an unwritten rule, law, agreement etc is one that everyone knows about although it is not official