A ruler, sometimes called a rule, scale or a line gauge or metre/meter stick, is an instrument used to make length measurements, whereby a length is read from a series of markings called "rules" along an edge of the device.^[1] Usually, the instrument is rigid and the edge itself is a straightedge ("ruled straightedge"), which additionally allows one to draw straighter lines.

Rulers have long been made from different materials and in multiple sizes. Historically, they were mainly wooden but plastics have also been used. They can be created with length markings instead of being scribed. Metal is also 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 in or 30 cm in length, although some can go up to 100cm, it is useful for a ruler to be kept on a desk to help in drawing. Shorter rulers are convenient for keeping in a pocket.^[2] Longer rulers, e.g., 46 cm (18 in), 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 Classically, long measuring rods were used for larger projects, now superseded by the tape measure, the surveyor's wheel or laser rangefinders.

In geometry, straight lines between points may be drawn using a straightedge (ruler without any markings on it). Furthermore, it is also used to draw accurate graphs and tables.

A ruler and compass construction is a construction that uses a ruler and a compass. It is possible to bisect an angle into two equal parts with a ruler and compass. It can be proven, 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, if 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.^[3] In the late 18th century the metric system came into use and has been adopted to varying degrees in almost all countries in the world.

The oldest preserved measuring rod is a copper-alloy bar that dates from c. 2650 BC and was found by the German Assyriologist Eckhard Unger while excavating at the Sumerian city of Nippur (present day Iraq).

Rulers made of ivory were in use by the Indus Valley civilization period prior to 1500 BC.^[4] Excavations at Lothal (2400 BC) have yielded one such ruler calibrated to about 1.6 millimetres (1⁄16 in).^[4] Ian Whitelaw holds that the Mohenjo-Daro ruler is divided into units corresponding to 33.5 millimetres (1.32 in) and these are marked out in decimal subdivisions with amazing accuracy, to within 0.13 millimetres (0.005 in). Ancient bricks found throughout the region have dimensions that correspond to these units.^[5]

Anton Ullrich invented the folding ruler in 1851. Frank Hunt later made the flexible ruler in 1902.^[6]

The equivalent of a ruler for drawing or reproducing a smooth curve, where it takes the form of a rigid template, is known as a French curve. A flexible device that 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.^[7]

Ludwig Wittgenstein famously used rulers as an example in his discussion of language games in the Philosophical Investigations (1953). 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. However, 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.

• Cherry, Dan. "Collector's guide to rules", Furniture & Cabinetmaking, no. 259, July 2017, ISSN 1365-4292, pp. 52–6
• Rees, Jane and Mark (2010). The Rule Book: Measuring for the Trades. Lakeville, MN: Astragal Press ISBN 978-1-931626-26-2 OCLC 907853704
• Russell, David R.; with photography by James Austin and foreword by David Linley (2010). Antique Woodworking Tools: Their Craftsmanship from the Earliest Times to the Twentieth Century, Cambridge: John Adamson ISBN 978-1-898565-05-5 OCLC 727125586, pp. 64–74
• Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan ISBN 0-312-37026-1 OCLC 938084552