The small dime-sized hole in an elevator door is a keyhole that allows the door to be opened during an emergency or for routine maintenance. A drop key is used to open the elevator door via the small hole, according to Slate.
The keyhole only appears on doors that open to the lobby or floor, not in the elevator itself, so people see the keyhole while waiting for a ride. There are two sets of doors: one that opens to the floor and elevator shaft and another that opens the actual lift car. The keyhole releases the outer set of doors, giving workers access to the shaft.
The drop key is made available to authorized personnel. The unlocking device is on the back of the door, and the hole allows worker access to the mechanism. Sometimes a keyhole is shaped like a half-moon because that is the shape of the key. Drop keys turn side-to-side while a half-moon key requires lifting motions to open the door.
After the key opens the door to the elevator, personnel need to push the doors sideways to access the shaft. Personnel need to get into the elevator's shaft for repairs, emergency rescues or routine inspections.