The Berlin key

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The Berlin key (also known as "Schließzwangschlüssel" (German) or "close constraint key" (English)) is a type of key lock. It was designed to force people to close and lock their doors (usually a front door or gate). Its particularity comes from the fact that it has two key tips (the part which activates the bolt), one at each end of the key, rather than the usual single tip. After unlocking the lock, the key must be retrieved on the other side of the door after it has been closed again. The mechanism makes the retrieval of the key impossible until the door has been locked again from the other side. With this type of locking system, locking the bolt while the door is open is normally not possible.

Invented by the Berliner locksmith Johann 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. However, it continues to be a distinctive feature of the city of Berlin, Germany.