A ruler, sometimes called a rule or line gauge, is an instrument used in geometry, technical drawing, printing as well as engineering and building to measure distances or to rule straight lines. The ruler is a straightedge which may also contain calibrated lines to measure distances.
Rulers have long been made of many materials in a wide range of sizes. Some are wooden. Plastics have also been used since they were invented; they can be molded with length markings instead of being scribed. Metal is used for more durable rulers for use in the workshop; sometimes a metal edge is embedded into a wooden desk ruler to preserve the edge when used for straight-line cutting. 12 inches or 30 cm in length is useful for a ruler to be kept on a desk to help in drawing. Shorter rulers are convenient for keeping in a pocket. Longer rulers, e.g., 18 inches (45 cm) are necessary in some cases. Rigid wooden or plastic yardsticks, 1 yard long and meter sticks, 1 meter long, are also used. Classically, long measuring rods were used for larger projects, now superseded by tape measure or laser rangefinders.
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.
A line gauge is a type of ruler used in the printing industry. These may be made from a variety of materials, typically metal or clear plastic. Units of measurement on a basic line gauge usually include inches, agate, picas, and points. More detailed line gauges may contain sample widths of lines, samples of common type in several point sizes, etc.
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-meter carpenter's rule which folds down to a length of 24 cm 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 centimeters. 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.
A contraction rule is made having larger divisions than standard measures to allow for shrinkage of a metal casting. They may also be known as a 'shrinkage or shrink rule.
A ruler software program can be used to measure pixels on a computer screen or mobile phone. These programs are also known as virtual rulers. 
Ruler applications in geometry
In geometry, a ruler without any marks on it (a straightedge) may be used only for drawing straight lines between points. A straightedge is also used to help draw accurate graphs and tables.
A ruler and compass construction refers to constructions using an unmarked ruler and a compass. It is possible to bisect an angle into two equal parts with ruler and compass. It can be proved, though, that it is impossible to divide an angle into three equal parts using only a compass and straightedge — the problem of angle trisection. However, should two marks be allowed on the ruler, the problem becomes solvable.
In the history of measurement many distance units have been used which were based on human body parts such as the cubit, hand and foot and these units varied in length by era and location. In the late 18th century the metric system came into use and has been adopted almost all countries in the world. Rulers made of Ivory were in use by the Indus Valley Civilization period prior to 1500 BC. Excavations at Lothal (2400 BC) have yielded one such ruler calibrated to about 1⁄16 inch (1.6 millimetres). Ian Whitelaw holds that the Mohenjo-Daro ruler is divided into units corresponding to 1.32 inches (33.5 millimetres) and these are marked out in decimal subdivisions with amazing accuracy, to within 0.005 inches (0.13 millimetres). Ancient bricks found throughout the region have dimensions that correspond to these units.
Anton Ullrich invented the folding ruler in 1851.
Curved and flexible rulers
The equivalent of a ruler for drawing or reproducing a smooth curve is known, where it takes the form of a rigid template, as a French curve. A flexible device which can be bent to the desired shape is known as a flat spline, or (in its more modern incarnation) a flexible curve. Historically, a flexible lead rule used by masons that could be bent to the curves of a molding was known as a lesbian rule.
Ludwig Wittgenstein famously used rulers as an example in his discussion of language games in the Philosophical Investigations. He pointed out that the standard meter bar in Paris was the criterion against which all other rulers were determined to be one meter long, but that there was no analytical way to demonstrate that the standard meter bar itself was one meter long. It could only be asserted as one meter as part of a language game.
Sticking to the Ruler's Marks
When using a ruler use the smallest mark as the first estimated digit. For example if a ruler's smallest mark is cm, and 4.5 cm is read, it is 4.5 (+-.1) or range 4.4 to 4.6 cm.
The overall length of a ruler may not be accurate within the degree of the smallest mark and the marks may be imperfectly spaced within each unit. With both of these errors in mind, reading between the marks (to 4.55) does not increase one's confidence in the overall reading and may make it worse.
- Dividing engine
- Golomb ruler
- Measuring rod
- Rolling ruler
- Scale rulers: architect's scale and engineer's scale
- Significant figures
- Accuracy and precision
- Dictionary.com definition
- "Steel Rule Has Pocket Clip For Use As A Depth Gauge" Popular Science, December 1935 pg.887 bottom right
- contraction rule Retrieved May 15, 2013
- Online ruler Retrieved May 13, 2014
- Klein, Herbert A.. The science of measurement: a historical survey. Reprint, unabridged, corr. republ. der Ausg. New York, Simon & Schuster, 1974. ed. New York, NY: Dover, 1988. Print.
- Whitelaw, p. 14.
- Whitelaw, p. 15.
- "lesbian, adj. and n.". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (subscription required)
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- Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan. ISBN 0-312-37026-1.