One must walk...softly...so as not to disturb the people in the apartment below.
One must not shout private & potentially embarrassing things to ones spouse if he is in another room (even if the apt. is small).
One cannot watch movies at full, heart pounding volume.
One cannot listen to The Vines, Green Day, or any other loud band at the volume which best suits loud band music.
One must remember that only 1 person can fit in the kitchen at a time.
Ditto on the bathroom
Rules. Lots and lots of rules.