Instruments for describing circles, measuring lines, etc. They are of various descriptions; the common consist of two sharp-pointed branches or legs, turning upon a hinge at top. Triangular compasses are constructed as the ordinary compasses, with the addition of a third leg turning upon a universal joint; they are used in laying down places on a chart, or measuring angles.

Beam Compasses are for the purpose of describing circles of considerable radius, and consist of a straight bar of wood or metal, of suitable length, furnished with a fixed point as a centre, and a movable tracing point, which may be set to any required radius.

Elliptical Compasses, likewise called a "trammel," for the purpose of describing ellipses, consist of a bar with a fixed tracing point, and two cursors, which can be set upon any part of the bar, and which have a sliding motion in two grooves cut in a piece of brass, which grooves intersect each other at right angles.

Proportional Compasses are composed of two flat bars pointed at the end, and having a groove down the middle, in any part of which can be set a steel pivot, held fast by a tightening screw, by which means the compasses can be divided into two parts, bearing any required proportion to each other, and which, turning upon the pivot in the manner of a pair of scissors, the angles at the opposite extremities will be proportional to the length of the respective parts.