A ruler, or rule, is an instrument used in geometry, technical drawing and engineering/building to measure distances and/or to rule straight lines. Strictly speaking, the ruler is essentially a straightedge used to rule lines and the calibrated instrument used for determining measurement is called a 'measure'. However, common usage is that a ruler is a calibrated straightedge that can be used for making measurements.
Desk rulers are used for three main purposes: to measure, to aid in drawing straight lines and as a straight guide for cutting and scoring with a blade. Practical rulers have distance markings along their edges.
Measuring instruments similar in function to rulers are made portable by folding (carpenter's folding rule) or retracting into a coil (metal tape measure) when not in use. When extended for use they are straight, like a ruler. The illustrations on this page show a 2 metre carpenter's rule which folds down to a length of 24 centimeters to easily fit in a pocket, and a 5 meter long tape which retracts into a small housing. A flexible length measuring instrument which is not necessarily straight in use is the tailor's fabric tape measure, a length of tape calibrated in inches and centimetres. It is used to measure around a solid body, e.g., a person's waist measurement, as well as linear measurement, e.g., inside leg. It is rolled up when not in use, taking up little space.
In geometry, a ruler usually refers to an unmarked ruler (a straightedge), only used for drawing straight lines between points, not measuring.
A ruler and compass construction refers to constructions using an unmarked ruler and a compass. It is possible to bisect and angle into 2 equal parts with ruler and compass. It can be proved, though, that it is impossible to divide an angle into 3 equal parts using only a compass and straightedge—the problem of angle trisection. However, should a single mark be allowed on the ruler, the problem becomes solvable..