BerlinKey

The Berlin key (German: Schließzwangschlüssel), also known as 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. The key was a solution to the problem of access via communal doors of such blocks (Mietskaserne) as early as the 19th century.