A rule, usually concerning
social behavior, which is known by all but spoken by
none. This rule is neither official nor written down. It just is.
Examples of what an Unwritten Rule is:
You do not
sit next to strangers on busses/trains even if it is full - you stand.
You do not stare at people in public.
You do not tell a girl she is fat, even if she is.
You never pass a cop even if he is doing 10 under.
You do not swear in the presence of a lady.
You do not initiate unwelcomed small talk with who you are sitting
next to on a plane.
You take your
hat off during the national anthem.
etc.