This wooden rule is known as a Gunter rule and is inscribed with scales to help with navigational calculations. The scales on the front include a linear scale in inches, a diagonal scale to set exact lengths, scales of leagues, chords, rhumbs, sines, tangents, semi-tangents, and miles of longitude. Those on the back include sines of rhumbs, tangents of rhumbs, a line of numbers, logsines, logversines, logtangents, meridional lines and equal parts. The maker's name, Thomas Arnold, and the date 1700 are both inscribed on the rule. This is unusual - these wooden rules are usually unsigned.

The Gunter rule was designed to help with the mathematical calculations necessary in navigation. The rule is intended to be used with a pair of dividers, so that the calculations are carried out by measuring off lengths against the different scales. It is named after Edmund Gunter (1581–1626), the inventor of the logarithmic scale, which he first described in 1623. Gunter rules were used in navigation into the 19th century.

This rule was one of a number of objects recovered from a wreck on the Goodwin Sands. The wreck was probably the ‘Stirling Castle’, which went aground during the Great Storm of 1703.