The sector, also known as a sector rule, proportional compass, or military compass, is a calculating instrument used throughout the sixteenth to nineteenth century. It consists of two rulers of equal length joined by a hinge. Several sets of scales are inscribed upon the instrument which enable various mathematical calculations.
The sector is used for solving problems in proportion, multiplication and division, geometry, and trigonometry, and also for computing various mathematical functions, such as square roots and cube roots. It is a precursor of the more modern slide-rule, and its scales permitted easy and direct solutions of problems in gunnery, surveying, and navigation. The sector is based on the fourth proposition of the sixth book of Euclid, whereby it is demonstrated that similar triangles have their like sides proportional.
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