Editing Chain (unit) (section)

{{Short description|Unit of length}}
{{Infobox unit
| name     = chain
| image = Gunter's chain at Campus Martius Museum.JPG
| caption = The chain is based on the length of Gunter's chain, which is {{convert|66|ft|yd}} long.
| standard = [[Imperial units|Imperial]]/[[US customary units|US]]&nbsp;units
| quantity = [[length]]
| symbol   = ch
| units1   = Imperial/US&nbsp;units
| inunits1 = {{val|22|u=[[Yard (unit)|yd]]}}, {{val|66|u=[[Foot (unit)|ft]]}}, {{val|100|u=[[Link (unit)|links]]}} 
| units2   = [[Metric system|Metric]] ([[SI]]) units
| inunits2 = {{convert|22|yd|m|4|lk=on|disp=out}}
}}

The '''chain''' (abbreviated '''ch''') is a [[Units of measurement|unit]] of [[length]] equal to 66 [[foot (unit)|feet]] (22 [[yard (unit of length)|yards]]), used in both the [[United States customary units|US customary]] and [[Imperial units|Imperial]] unit systems. It is subdivided into 100 [[Link (unit)|links]].<ref>{{cite book |title=Mathematics Dictionary |page= 453 |first1=Robert C. |last1=James |first2=Glenn |last2=James |isbn=9780412990410 |publisher=Chapman & Hall |location=New York |date=1992}}</ref><ref name="USNBS1960">{{cite book|title=Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents|url=https://nvlpubs.nist.gov/nistpubs/Legacy/MP/nbsmiscellaneouspub214.pdf|year=1960|publisher=U.S. Department of Commerce, National Bureau of Standards|pages=8–9|chapter=6 Tables of Interrelation of Units of Measurement}} (PDF)</ref> There are 10 chains in a [[furlong]], and 80 chains in one [[statute mile]].<ref name="USNBS1960" /> In metric terms, it is 20.1168&nbsp;[[metre|m]] long.<ref name="USNBS1960" /> By extension, '''chainage''' (running distance) is the distance along a curved or straight survey line from a fixed commencing point, as given by an [[odometer]].

The chain has been used since the early 17th century in England, and was brought by British settlers during the colonial period to other countries around the globe. In the United Kingdom, there were 80 chains to the mile, but until the early nineteenth century the [[Mile#British and Irish miles|Scottish and Irish customary miles]] were longer than the [[statute mile]]; consequently a Scots chain was about 74 (imperial) feet,<ref>{{cite book |author-link=John Smeaton |last=Smeaton |first=John |title=Reports of the Late John Smeaton, F.R.S. |date=1837 |publisher=M Taylor |location=London |page=[https://archive.org/details/reportslatejohn00smeagoog/page/n354 308] |edition=2nd |url=https://archive.org/details/reportslatejohn00smeagoog |quote=Since the foregoing Report [on the best route for the [[Forth and Clyde Canal]]] ... was delivered ... , Mr Smeaton has discovered that, notwithstanding the care and pains he took to be correct, he has committed an error, in supposing the Scotch chain, with which the measures of the length of the tract of land were taken, to consist of seventy feet each, whereas, in reality, it consists of seventy-four}}</ref> an Irish chain 84 feet. These longer chains became obsolete following the adoption of [[Imperial units|the imperial system of units]] in 1824.<ref>{{cite book |author1=William John Macquorn Rankine |author-link=William John Macquorn Rankine |title=A Manual of Civil Engineering |date=1863 |publisher=Griffin Bohn & Company |location=London |page=[https://archive.org/details/amanualcivileng05rankgoog/page/n368 3] |edition=2nd |url=https://archive.org/details/amanualcivileng05rankgoog}}</ref> In India, "metric chains" of exactly {{convert|20|metres|2|abbr=off}} are used, along with fractions thereof.<ref name=Punmia_etal />