View source for Ruler

{{Short description|An instrument used to measure distances or to draw straight lines}}
[[File:Diverse·Maßstäbe,15.365.jpg|thumb|right|upright=1.3|A variety of rulers]]
[[File:CarpentersRule.png|thumb|right|A {{cvt|2|m|ftin|lk=on|frac=8}} 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 or a '''line gauge''', is an instrument used to make [[Length measurement|length measurement]]s, whereby a user estimates a length by reading 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> Commonly the instrument is rigid and the edge itself is a [[Straightedge|straightedge]] ("ruled straightedge"), which additionally allows one to draw straight lines. Some rulers, such as cloth or paper [[Engineering:Tape measure|tape measure]]s, are non-rigid. Specialty rulers exist that have flexible edges that retain a chosen shape; these find use in sewing, arts, and crafts.

Rulers have been used since ancient times. They are commonly made from metal, wood, fabric, paper, and plastic. They are important tools in the design and construction of buildings. Their ability to quickly and easily measure lengths makes them important in the textile industry and in the retail trade, where lengths of string, fabric, and paper goods can be cut to size. Children learn the basic use of rulers at the elementary school level, and they are often part of a student's school supplies. At the high school level rulers are often used as straightedges for geometric constructions in [[Euclidean geometry]]. Rulers are ubiquitous in the engineering and construction industries, often in the form of a tape measure, and are used for making and reading [[Engineering:Technical drawing|technical drawing]]s. Since much technical work is now done on computer, many software programs implement virtual rulers to help the user estimate virtual distances.

==Variants==
[[File:Gilded Bronze Ruler - 1 chi = 231 cm. Western Han (206 BCE - CE 8). Hanzhong City.jpg|thumb|left|Gilded bronze ruler. 1&nbsp;chi = {{cvt|231|mm}}. Western Han (206&nbsp;BC – 8&nbsp;AD). Hanzhong City, China]]
[[File:Han Bronze Ruler (9948006553).jpg|thumb|left|Bronze ruler. Han dynasty, 206&nbsp;BC – 220&nbsp;AD. Excavated in Zichang County, China|221x221px]]
Rulers have long been made from different materials and in multiple sizes. Historically they were mainly wooden; but 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. {{cvt|12|in|cm|lk=on|disp=or}} 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.<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|order=flip}}, are necessary in some cases. Rigid wooden or plastic yardsticks, 1&nbsp;yard long, and meter sticks, 1&nbsp;meter long, are also used. Classically, long [[Engineering:Measuring rod|measuring rod]]s were used for larger projects, now superseded by [[Engineering:Tape measure|tape measure]], [[Engineering:Surveyor's wheel|surveyor's wheel]] 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.
<!-- How these distance markings are applied and calibrated should be described here, including a history of old methods. -->

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 (typography)|agate]], [[Pica (typography)|picas]], and [[Point (typography)|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 {{cvt|2|m|ftin}} carpenter's rule, which folds down to a length of {{cvt|25|cm|in|0}} to easily fit in a pocket, and a {{cvt|5|m|ft|0}} 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. for a person's waist measurement, as well as for linear measurement, e.g., for the inside leg length. It is rolled up when not in use, taking up little space.

An ''Online ruler software program'' can be used to measure pixels on a computer screen or mobile phone. These programs are also known as screen rulers or online ruler.<ref>{{cite web|url=https://onlineruler.org/|title=Online Ruler}}</ref> It is used to measure the actual size of an object in inches, centimeters, or millimeters.

== Use 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 a 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&nbsp;— the problem of [[Angle trisection|angle trisection]]. However, should two marks be allowed on the ruler, the problem becomes solvable.

==History==
[[File:Nippur-Elle (Top).jpg|thumb|upright=1.5|The Nippur cubit-rod, {{circa|2650 BC}}, in the 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 [[Engineering:Carrack|carrack]] ''Mary Rose'']]

In the [[History of measurement|history of measurement]] many distance units have been used which were based on human body parts such as the [[Cubit|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 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 {{circa}} 2650&nbsp;BC and was found by the German Assyriologist Eckhard Unger while excavating at the Sumerian city of [[History:Nippur|Nippur]] (present day Iraq). 

Rulers made of ivory were in use by the Indus Valley civilization period prior to 1500&nbsp;BC.<ref name="Whitelaw14"/> Excavations at [[Earth:Lothal|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 [[Engineering:Flat spline|flat spline]], or (in its more modern incarnation) a ''flexible curve''. Historically, a flexible [[Chemistry:Lead|lead]] rule used by masons that could be bent to the curves of a [[Engineering:Molding (decorative)|molding]] was known as a [[Engineering:Lesbian rule|lesbian rule]].<ref name=oed>{{OED|lesbian rule}} {{subscription}}</ref>

==Philosophy==
[[Biography:Ludwig Wittgenstein|Ludwig Wittgenstein]] famously used rulers as an example in his discussion of language games in the ''[[Philosophy:Philosophical Investigations|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, 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.
<!-- 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==
{{div col|colwidth=30em}}
* {{annotated link|Accuracy and precision}}
* {{annotated link|Engineering:Dividing engine}}
* {{annotated link|Golomb ruler}}
* {{annotated link|Engineering:Measuring instrument}}
* {{annotated link|Engineering:Measuring rod}}
* Scales: 
* {{annotated link|Significant figures}}
* {{annotated link|Engineering: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 }}
* Russell, David R.; with photography by James Austin and foreword by David Linley (2010). ''[[Engineering:Antique Woodworking Tools|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.&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}}


{{Measuring and alignment tools}}
{{Metalworking navbox|toolopen}}

[[Category:Metalworking measuring instruments]]
[[Category:Stonemasonry tools]]
[[Category:Woodworking measuring instruments]]
[[Category:Mathematical tools]]

{{Sourceattribution|Ruler}}