What kind of rules exists?
The rules are different from hostel to hostel, and in the end it is all about common sense and respect . In a hostel you usually are given a lot of freedom, don’t misuse it. If you come home to the hostel in the middle of the night, try not to make too much a fuss about getting into bed (a flashlight becomes very handy!), if there is a queue for the showers, consider wheter or not you actually should let the shampoo work for 6 minutes before rinsing it out. Upon arrival, ask the hostel staff what rules they have.
