The Shift lock key was introduced so the shift operation could be maintained indefinitely without continuous effort. It mechanically locked the typebars in the shifted position, causing the upper character to be typed upon pressing any key. Because the two shift keys on a typewriter required more force to operate and were meant to be pressed by the little finger, it could be difficult to hold the shift down for more than two or three consecutive strokes, therefore the introduction of the Shift lock key was also meant to reduce finger muscle pain caused by repetitive typing.