Sector rues were in vogue for solving all manner of trigonometric and navigational problems from the 1700s through the invention of the Gunter and Slide Rules in the next century. This rule, with its “sector” lines that radiate from the hinge joint, and require a fine set of dividers for use, was an essential tool for engineering and navigation, and was included in every kit of instruments. This example is unmarked (as are most), but surely was made in England, most likely London, in the 19th century. It is clean and white, with bold markings and has no cracks at the hinge as do most of the examples that you will see. This one has edge graduations. Folded, it is 6 inches long and has a single round brass joint with steel rivet. It is fine.