" Less familiar (now) is the telephone switchboard, for which an operator had to be able to selectively connect together pairs of the thousands of possibilities, as quickly as possible.
Charles E. Scribner, a man of great ingenuity, developed the “jack-knife switch” to make fast switching possible. This was a plug with a jackknife-like handle (hence the name) that could be inserted into a socket to activate the switch. Two such plugs at either ends of a cord allowed the connection of any two sockets, yielding an active telephone connection.