The parallel rule was designed to assist in drawing parallel lines accurately. For navigators, this is particularly important in laying off a specific course on a chart and parallel rules are still used today. They were invented in 1584 by a Frenchman, Mordente, but do not seem to have become common in navigation until the 18th century. This example is made of ebony with brass fittings. It was originally used by Vice-Admiral the Hon. A. C. Stonatt, when he was naval officer of HMS ‘Lion’ during battle of Jutland. He later became Master of the Grand Fleet.