Navigators use parallel rules for laying off a specific course on a chart. They were first invented in 1584 by a Frenchman named Mordente, but only became common in navigation in the 18th century.

Captain Feild's (or Field's) design had one of its outer edges divided into degrees like a protractor, with the opposite edge divided into points, half-points and quarter-points (of the compass). This made it easier to read off the bearing of a line than with a plain parallel rule, which had to be moved across the chart to the nearest compass diagram. This example has two inscriptions: 'CAPTN. FIELD'S IMPROVED' and 'REGD. NO. 3549 JANY 1854'. The registered design names the maker as J.D. Potter, then working at 31 Poultry, London, although no maker's name is given on this example.