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.