The parallel rule was designed to assist in drawing parallel lines accurately. For navigators, this is important in laying off a specific course on a chart and parallel rules are still used today. This example is made of ebony and brass. The maker’s name is inscribed on the top of one of the limbs as ‘THOMAS JONES CHARING CROSS LONDON’, with ‘S 260’ inscribed on the other limb. The latter is a mark from the Hydrographic Office, which introduced a system for marking its instruments in 1828. S indicated scales, parallel rules and box scales.