View source for Ruler

{{Short description|Instrument used to measure distances}}
{{other uses}}
{{Redirect|Carpenter's rule|the mathematical unfolding problem|Carpenter's rule problem}}
{{More citations needed|date=November 2023}}
[[File:Diverse·Maßstäbe,15.365.jpg|thumb|right|upright=1.3|A variety of rulers]]
[[File:CarpentersRule.png|thumb|right|A carpenter's rule]]
[[File:Measuring-tape.jpg|thumb|right|Retractable flexible rule or [[tape measure]]]]
[[File:Steel ruler closeup.jpg|thumb|right|A closeup of a steel ruler]]
[[File:Briefwaage Lineal.jpg|thumb|A ruler in combination with a letter scale]]

A '''ruler''', sometimes called a '''rule''', '''scale''', '''line gauge''', or '''metre/meter stick,''' is an instrument used to make [[length measurement]]s, whereby a length is read from a series of markings called "rules" along an edge of the device.<ref>{{cite web |title=ruler noun - Definition, pictures, pronunciation and usage notes - Oxford Advanced Learner's Dictionary at OxfordLearnersDictionaries.com |website=www.oxfordlearnersdictionaries.com |url=https://www.oxfordlearnersdictionaries.com/definition/english/ruler |access-date=25 April 2018 |url-status=live |archive-url=https://web.archive.org/web/20171025132533/http://www.oxfordlearnersdictionaries.com/definition/english/ruler |archive-date=25 October 2017}}</ref> Usually, the instrument is rigid and the edge itself is a [[straightedge]] ("ruled straightedge"), which additionally allows one to draw straighter lines. Rulers are an important tool  in geometry, geography and mathematics. They have been used since at least 2650 BC.

==Variants==
Rulers have long been made from different materials and in multiple sizes. Historically, they were mainly [[wood]] but [[plastics]] have also been used. They can be created with length markings instead of being [[wikt:scribe|scribed]]. Metal is also used for more durable rulers for use in the workshop; sometimes a raised metal edge is embedded into a wooden desk ruler to preserve the edge when used for straight-line cutting and to prevent ink from feathering under the ruler when used to draw a straight line with a pen. Typically {{cvt|12|in|cm|lk=on}} in length, though some can go up to 100 cm, it is useful for a ruler to be on a desk or workstation to help in drawing. Shorter rulers are convenient for keeping in a pocket.<ref>[https://books.google.com/books?id=x98DAAAAMBAJ&pg=PA887 "Steel Rule Has Pocket Clip For Use As A Depth Gauge"], ''Popular Science'', December 1935, p. 887 bottom right.</ref> Longer rulers (e.g. {{cvt|18|in|cm}}) are necessary in some cases, some examples being the  [[yardstick]]s and [[meter stick]]s. Historically, long [[measuring rod]]s were used for larger projects, now superseded by the [[tape measure]], [[surveyor's wheel|the surveyor's wheel]] or laser [[Rangefinding telemeter|rangefinder]]s.

== Use in geometry ==
{{Main article|Compass and straightedge}}

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

A [[compass and straightedge|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&nbsp;— the problem of [[angle trisection]]. However, if two marks be allowed on the ruler, the problem becomes solvable via a [[neusis]] construction.

==History==
[[File:Nippur-Elle (Top).jpg|thumb|upright=1.5|The Nippur cubit-rod, {{circa|2650 BC}}, in the [[Istanbul Archaeology Museums|Archeological Museum]] of [[Istanbul]], Turkey]]
[[File:MaryRose-carpentry tools1.jpg|right|thumb|A wooden carpenter's rule and other tools found on board the 16th-century [[carrack]] ''[[Mary Rose]]'']]

In the [[history of measurement]] many distance units have been used which were based on the human body such as the [[cubit]], [[Hand (unit)|hand]] and [[Foot (unit)|foot]], and these units varied in length by era and location.<ref>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.</ref> In the late 18th century, the [[Metrication|metric system]] came into use and has since been adopted to varying degrees in almost all countries around the world. 

The oldest preserved measuring rod is a copper-alloy bar that dates from {{circa}} 2650&nbsp;BC and was found by the German Assyriologist [[Eckhard Unger]] while excavating at the [[Sumer|Sumerian city]] of [[Nippur]] (present-day Iraq).{{Citation Needed|date=May 2025}}

Rulers made of [[ivory]] were in use by the [[Indus Valley civilization]] period prior to 1500&nbsp;BC.<ref name="Whitelaw14"/> Excavations at [[Lothal]] (2400&nbsp;BC) have yielded one such ruler calibrated to about {{convert|1/16|inch|mm|1|order=flip}}.<ref name="Whitelaw14">Whitelaw, p. 14.</ref> Ian Whitelaw holds that the [[Mohenjo-Daro]] ruler is divided into units corresponding to {{convert|1.32|inch|mm|1|order=flip}} and these are marked out in decimal subdivisions with amazing accuracy, to within {{convert|0.005|in|mm|order=flip}}. Ancient bricks found throughout the region have dimensions that correspond to these units.<ref>Whitelaw, p. 15.</ref>

[[Anton Ullrich]] invented the folding ruler in 1851. Frank Hunt later made the flexible ruler in 1902.<ref>{{cite web|title=Flexible Ruler Invented by Frank G. Hunt|url=http://americanhistory.si.edu/collections/search/object/nmah_1214954|website=National Museum of National History|access-date=7 June 2016|url-status=live|archive-url=https://web.archive.org/web/20160630014738/http://americanhistory.si.edu/collections/search/object/nmah_1214954|archive-date=30 June 2016}}</ref>

==Curved and flexible rulers==
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 [[masonry|masons]] that could be bent to the curves of a [[Molding (decorative)|molding]] was known as a [[lesbian rule]].<ref name=oed>{{OED|lesbian rule}} {{subscription required}}</ref>

==Philosophy==
[[Ludwig Wittgenstein]] famously used rulers as an example in his discussion of [[Language-game|language games]] in the ''[[Philosophical Investigations]]'' (1953).  He pointed out that the [[Metre#International prototype metre bar|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.
<!-- I know there are other philosophical usages out there; if nothing else Masonic theory loved rulers and measurement, so there's probably stuff from Ben Franklin.  Can anyone flesh this out? -->

==See also==
{{Portal|science}}
{{div col|colwidth=30em}}
* {{annotated link|Accuracy and precision}}
* {{annotated link|Dividing engine}}
* {{annotated link|Golomb ruler}}
* {{annotated link|Measuring instrument}}
* {{annotated link|Measuring rod}}
* {{annotated link|Rolling ruler}}
* Scales: 
**{{annotated link|Architect's scale}} 
**{{annotated link|Metric scale}} and 
**{{annotated link|Engineer's scale}}
* {{annotated link|Significant figures}}
* {{annotated link|Technical drawing tool}}
{{div col end}}

==References==
{{Reflist}}

==Bibliography==
* Cherry, Dan. "Collector's guide to rules", ''Furniture & Cabinetmaking'', no. 259, July 2017, [https://www.worldcat.org/ISSN/1365-4292 ISSN 1365-4292], pp.&nbsp;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 }}
* [[David R. Russell|Russell, David R.]]; with photography by [[James Austin (photographer)|James Austin]] and foreword by [[David Armstrong-Jones, Viscount Linley|David Linley]] (2010). ''[[Antique Woodworking Tools|Antique Woodworking Tools: Their Craftsmanship from the Earliest Times to the Twentieth Century]]'',  Cambridge: [[John Adamson (publisher)|John Adamson]] {{ISBN|978-1-898565-05-5}} {{OCLC|727125586}}, pp.&nbsp;64–74 
* Whitelaw, Ian (2007). ''A Measure of All Things: The Story of Man and Measurement''. Macmillan {{ISBN|0-312-37026-1}} {{OCLC|938084552}}

{{Wiktionary}}
{{Commons category|Length measuring devices}}

{{Measuring and alignment tools}}
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[[Category:Length, distance, or range measuring devices]]
[[Category:Metalworking measuring instruments]]
[[Category:Stationery]]
[[Category:Stonemasonry tools]]
[[Category:Woodworking measuring instruments]]
[[Category:Mathematical tools]]