Editing System of units of measurement (section)

==History==
{{see also|Units of measurement#History}}
{{further|History of measurement}}

In antiquity, ''systems of measurement'' were defined locally: the different units might be defined independently according to the length of a king's thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size, perhaps itself defined in ''hands'' and ''knuckles''. The unifying characteristic is that there was some definition based on some standard. Eventually ''[[cubit]]s'' and ''[[yard|strides]]'' gave way to "customary units" to meet the needs of merchants and scientists.

The preference for a more universal and consistent system only gradually spread with the growth of international trade and science. Changing a measurement system has costs in the near term, which often results in resistance to such a change. The substantial benefit of conversion to a more rational and internationally consistent system of measurement has been recognized and promoted by scientists, engineers, businesses and politicians, and has resulted in most of the world adopting a commonly agreed metric system.

The [[French Revolution]] gave rise to the [[metric system]], and this has spread around the world, replacing most customary units of measure. In most systems, [[length]] (distance), [[mass]], and [[time]] are ''base quantities''.

Later, science developments showed that an electromagnetic quantity such as [[electric charge]] or electric current could be added to extend the set of base quantities. [[Gaussian units]] have only length, mass, and time as base quantities, with no separate electromagnetic dimension. Other quantities, such as [[Power (physics)|power]] and [[speed]], are derived from the base quantities: for example, speed is distance per unit time. Historically, a wide range of units was used for the same type of quantity.  In different contexts length was measured in [[inch]]es, [[Foot (length)|feet]], [[yards]], [[fathom]]s, [[Rod (unit)|rods]], [[Chain (unit)|chains]], [[furlong]]s, [[mile]]s, [[nautical mile]]s, [[stadia (length)|stadia]], [[League (unit)|leagues]], with conversion factors that were not based on power of ten.

In the metric system and other recent systems, underlying relationships between quantities, as expressed by formulae of physics such as [[Newton's laws of motion]], is used to select a small number of base quantities for which a unit is defined for each, from which all other units may be derived. Secondary units (multiples and submultiples) are derived from these base and derived units by multiplying by powers of ten.  For example, where the unit of length is the [[metre]]; a distance of 1&nbsp;metre is 1,000&nbsp;millimetres, or 0.001&nbsp;kilometres.

===Current practice===
{{main article|Metrication}}
Metrication is complete or nearly complete in most countries.

However, [[US customary units]] remain heavily used in the [[United States]] and to some degree in [[Liberia]]. Traditional [[Burmese units of measurement]] are used in [[Burma]], with partial transition to the metric system. U.S. units are used in limited contexts in Canada due to the large volume of trade with the U.S.  There is also considerable use of imperial weights and measures, despite ''de jure'' Canadian conversion to metric.

A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom whose [[road signage legislation (UK)|road signage legislation]], for instance, only allows distance signs displaying [[imperial units]] (miles or yards)<ref name=TSRGD>{{cite web
|url = http://www.opsi.gov.uk/si/si2002/20023113.htm
|title = Statutory Instrument 2002 No. 3113 The Traffic Signs Regulations and General Directions 2002
|publisher = Her Majesty's Stationery Office (HMSO)
|year = 2002
|access-date = 18 March 2010}}</ref> or Hong Kong.<ref name="HK">[http://www.legislation.gov.hk/blis_ind.nsf/d2769881999f47b3482564840019d2f9/ca7c0e7895c7f088c82564760077b0c9?OpenDocument HK Weights and Measures Ordinance]</ref>

In the United States, metric units are virtually always used in science, frequently in the military, and partially in industry.  U.S. customary units are primarily used in U.S. households. At retail stores, the litre (spelled 'liter' in the U.S.) is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather than [[Grain (unit)|grains]], are used for medications. 

Some other non-[[SI]] units are still in international use, such as [[nautical mile]]s and [[knot (unit)|knots]] in aviation and shipping, and [[Foot (unit)|feet]] for aircraft altitude.