Sector, made of ivory, with brass hinges and brass studs at key points to prevent damage when being used with a pair of dividers. The sector was a calculating instrument commonly used in navigation and surveying. The relatively late date of this example shows the persistence of such devices.

Scales on the front include: Inches (0-12); L (equal parts); S (secants); C (chords); POL (polygons).
Scales on the back include: S (sinnes); T (tangents); N (line of numbers).
On the edge is a linear scale marked 0-100.