Sector

A brass sector (length closed 5.1 in., open 9.4 in.) features two logarithmically scaled arms joined at a pivotal hinge. Although no numbers are present, recessed dots or needle holes in sets of two, three, or four bracket some of the line spaces.

A line of English inches (2.54 cm) occurs on the inside edge of one arm, while the back edge when opened is divided evenly into eight French inches (2.75 cm); the first block of both scales is further divided into 12 increments. A small set pin ensures that the arms line up when the instrument is closed. When used with a pair of compasses, the sector enabled problems involving proportions to be solved instrumentally.