The
sector, also known as a
proportional compass or
military compass, was a major calculating instrument in use from the end of the sixteenth century until the nineteenth century. It is an instrument consisting of two rulers of equal length joined by a hinge. A number of scales are inscribed upon the instrument which facilitate various mathematical calculations. It was used for solving problems in
proportion,
trigonometry,
multiplication and
division, and for various functions, such as
squares and
cube roots. Its several scales permitted easy and direct solutions of problems in
gunnery,
surveying and
navigation. The sector derives its name from the fourth proposition of the sixth book of
Euclid, where it is demonstrated that similar
triangles have their like sides proportional. It has four parts, two legs with a pivot (the articulation), a quadrant and a clamp (the curved part at the end of the leg) that enables the compass to function as a gunner's quadrant.