In geometry, geometrical construction is the process of drawing a geometrical figure using only two instruments - a straight edge (or ungraduated scale) and a compass. If measurements are required in construction, then we may use a ruler (which is graduated) and a protractor.
Normally, in the drawing of geometrical figures such as lines, triangles, quadrilaterals, polygons and circles, etc. with given measurements, following instruments are used:
- Compass: A compass is a drawing instrument used for drawing circles and arcs. It is typically made of metal or plastic, and consists of two legs connected by a hinge, one with a point and the other with a pencil or lead. You can adjust the distance between the point and the pencil and that setting will remain until you change it.
- Protractor: It is a measuring instrument, typically made of transparent plastic or glass, for measuring angles. Most of the protractors measure angles in degrees (°). Some of the protractors are half-circle protractors to measure angles up to 180°. Others are full-circle protractors to measure angles up to 360°.
- Ruler: It is an instrument used to measure distances or to draw straight lines. It is graduated in centimeters and millimeters on one side. The other side has inches sub-divided into 1/16th of an inch. The ruler is also commonly called as a "scale".
- Straightedge: It is simply a guide for the pencil when drawing straight lines. It doesn't contain equally spaced markings on both sides.
- Set-square: It is a right-angled triangular plate for drawing lines especially at 90°, 45°, 60° or 30°. There are two types of set-squares: one with angles 90°, 60° and 30° and other with angles 90°, 45° and 45°.
- Divider: It is an instrument for measuring, transferring, or marking of distances. It consists of two straight adjustable legs hinged together and ending in sharp points.