page_title: Chain (unit)
section_title: Britain
hierarchical_section_title: Chain (unit) / Modern use and historic cultural references / Britain
caption_attribution_description: English: Miles and chains used as a location designator on a British railway bridge
context_page_description: The chain is a unit of length equal to 66 feet. It is subdivided into 100 links or 4 rods. There are 10 chains in a furlong, and 80 chains in one statute mile. In metric terms, it is 20.1168 m long. By extension, chainage 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 for several centuries in England and in some other countries influenced by English practice. In the United Kingdom, there were 80 chains to the mile, but until the early nineteenth century the Scottish and Irish customary miles were longer than the statute mile; consequently a Scots chain was about 74 feet, an Irish chain 84 feet. These longer chains became obsolete following the adoption of the imperial system in 1824.
context_section_description: In Britain, the chain is no longer used for practical survey work. However it survives on the railways of the United Kingdom as a location identifier. When railways were designed, the location of features such as bridges and stations was indicated by a cumulative longitudinal "mileage", using miles and chains, from a zero point at the origin or headquarters of the railway, or the originating junction of a new branch line. Since railways are entirely linear in topology, the "mileage" or "chainage" is sufficient to identify a place uniquely on any given route. Thus a certain bridge location may be indicated as 112 miles and 63 chains (181.51 km) from the origin. In the case of the photograph the bridge is near Keynsham, that distance from London Paddington station. The indication "MLN" after the mileage is the engineers' line reference describing the route as the Great Western Main Line, so that visiting engineers can uniquely describe the bridge they are inspecting, as there may be bridges at 112 miles 63 chains on other routes.
  On new railway lines built in the United Kingdom such as High Speed 1, the position along the alignment is still called "chainage" although the value is now defined in metres.
caption: Location designator painted on a British railway bridge, showing 112 miles and 63 chains; photograph taken August 2007
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