When there is a sock on the door knob, this serves as a notification that the people inside this room or apartment are having sex.

Most common in roommate situations where the person you share the house/apartment/dorm/etc with are having sexual relations and request that they be left alone. A sock on the door knob makes a quick and easy 'do not disturb' sign. It also may be used for a reminder of some sort.

If you approach a room with a sock on the door knob (or possibily a rubber band), please please please do not knock or otherwise disturb the occupants. Please do not call the occupants on the phone. They obviously have something better to do than see you. GO AWAY!