The Berlin key

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The Berlin key is a very specific kind of key lock and is a distinctive feature of the city of Berlin, Germany. It particularity comes from its two key tips (the part which activate the bolt). After unlocking the lock, the key must be retrieved on the other side of the door to close this latter. Also called "Schließzwangschlüssel" or close constraint key in English. This key was actually designed to force people to lock their door (usually a front door or gate). The mechanism makes the retrieval of the key impossible once the bolt is unlocked, the door must be locked again in order to extract the key. With this type of key lock system, "locking" the door while the door is open is normally not possible.

Invented by the Berliner locksmith Johann Schweiger, the key was massively produced by the Alber Kerfin & Co company starting in 1912. Nowadays this kind of lock is disappearing but still is one of the typical object from Berlin.