For many years the professional piano players could rightfully object to the somewhat unelastic touch of the Janko keyboard. This objection was completely overcome by an ingenious improvement accomplished by Paul Perzina of Schwerin, who changed the double leverage of the key successfully to a single movement assuring the desired elastic touch. In order to facilitate the attachment of the Janko keyboard, Perzina invented a reversible double key-bottom, so that the Janko as well as the old style keyboard can be used on the same piano.
From the book Pianos and Their Makers
by Alfred Dolge
Originally published in 1911