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 mathematician Fabrizio Mordente, but do not seem to have become common in navigation until the 18th century. This rule is made of ivory with brass fittings and has six scales engraved on the underside. It operates by means of brass scissored links, on which the maker’s name is engraved.