Unwritten rules can also be explained as what is socially accepted or as social codes of conduct. That is, how you are expected to behave together with other people. An unwritten rule is a rule that is not described in any law but that people are expected to follow. A person who does not follow the unwritten rules can be regarded as ‘abnormal’ since that person is then breaching the norm. Examples are that you have to reply when someone addresses you, have to say hello back when someone says hello to you and that you have to behave politely to other people.
Read the book
Den Svenska Koden (The Swedish Code) by Silvia Nilsson Puccio, Uli Bruno & Maria Bengts.