File:Compas de proportion 1.jpg

English: Typical English sector made by the Dollond family firm of London, probably circa early 19th century, constructed of ivory with a brass hinge. This side shows four double scales, repeated on each of the sector's two legs. These scales are the line of lines (labeled L, marked from 1 to 10), a linear scale used for general proportion problems; the line of secants (labeled S, marked in degrees from 20 to 75), for finding the secant of a given angle; the line of chords (labeled C, marked in degrees from 10 to 60), for finding a given angle from a circle of known radius; and the line of polygons (labeled POL, marked from 12 out to 4), for inscribing within a circle a regular-sided polygon of the given number of sides. The L and C scales have their zero point at the sector's hinge; the S scales' zero point is at the 2.5 point on the L scales. Except for the POL scale, all the double scales are arranged so that they form the same-sized angle when the sector is opened a given amount, to make it easier to perform calculations that need more than one scale. A split scale of 12 inches is also present along the outer edges.