The Berlin key (also known as, German, Schließzwangschlüssel, or, English, forced locking key) is a key for a type of door lock. It was designed to force people to close and lock their doors, usually a main entrance door or gate leading into a common yard or tenement block.
A Berlin key has two key blades, at each end of the key, rather than the usual single blade. The key is used like this:
1. Unlock the door from the outside
2. Push the key through the lock, so that the key protrudes from the inside of the door
3. Open the now unlocked door and enter
4. Close the door and lock the door with the key, which now protrudes from the inside of the door
5. Take the key from the lock
The mechanism makes impossible to forget to lock the door, without also forgetting the key in the lock. Also, locking an open door is usually not possible.
Invented by the Berliner locksmith Johannes Schweiger, the Berlin key was massively produced by the Albert Kerfin & Co company starting in 1912. With the advent of more recent locking technologies, this kind of lock and key is becoming less common, but it can still be widely found in the tenement buildings of Berlin, Germany.