Do Not Disturb Signs

DND or Do not disturb is a standard phrase traditionally seen on doorknob hangers in group accommodations, such as hotels or college dorms. These are known as "Do not disturb" signs.
A do not disturb sign is a sign placed on a door, either hanging on the doorknob or in some other manner, that lets others know not to enter.
One common place where do not disturb signs are used is in places of lodging, where guests can place these signs on the door in order to inform staff, including housekeeping to respect their privacySome hotels also provide "make up room" signs giving the opposite instructions.
They are also used in schools to let other staff members and students at the school know that testing is in progress in the particular classroom.