The striking system known by the German term Sürrerwerk ('whizzing work') has been recorded in Italian and German clocks during the 18th and 19th centuries, and variants of the device are known from the 17th century. The system is based on the use of pins of graduated length mounted on opposite sides of a single pinwheel. These pins lift the two hammers to strike the hours and quarters, six on one side operating the hour hammer, the arbour of which is axially changed in position by a stepped-face cam to engage each of the pins progressively as the hours go by. The three pins on the other side of the pinwheel operate in a similar way to strike the quarters progressively by one, two and three strokes, the pinwheel making one complete revolution each quarter of an hour. In a full day the system thus strikes 228 blows in the following way: hours, 1+2+3+4+5+6x4 = 84 (a six-hour system of striking); quarters, 1+2+3x24 = 144. Grande sonnerie striking by this system is known; it strikes 864 strokes per day.