This instrument has 2 graduated rulers joined at one end by a hinge. When they are fully opened out they form a straight, double-length ruler. Various scales are engraved on the compass faces: planes, polygons, equal parts, strings, solids, and metals.
This type of mathematical instrument was mainly used to solve navigational problems involving flat and spherical triangles. A compass was also required to use it correctly. The book that explained how to solve these problems was "Description and Use of the Sector, the Crosse-staffe, and other Instruments" published in 1623 by English mathematician and astronomer Edmund Gunter (1581–1626).