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[[Image:SI Brochure Cover.jpg|frame|right|Cover of brochure ''[http://www.bipm.org/en/publications/brochure/ The International System of Units]''.]]
The '''International System of Units''' (abbreviated '''SI''' from the [[French language|French]] ''{{lang|fr| '''S'''ystème '''i'''nternational d'unités}}''<ref>[http://www.bipm.org/en/CGPM/db/11/12/ Bureau International des Poids et Mesures]</ref>) is the modern form of the [[metric system]] and is generally a system of [[units of measurement]] devised around seven [[SI base unit|base units]] and the convenience of the number ten. It is the world's most widely used [[Systems of measurement|system of measurement]], both in everyday [[commerce]] and in [[science]].<ref>[http://www.bipm.org/en/si/base_units/ Official BIPM definitions]</ref><!--English units are still used in some scientific applications, but note also that parsecs in astronomy, calories and mmHg in the medical sciences, and electron volts in physics are not part of the specific system of units known as SI, to just scratch the surface--><ref>An extensive presentation of the SI units is maintained on line by [http://www.physics.nist.gov/cuu/Units/units.html NIST], including a [http://www.physics.nist.gov/cuu/Units/SIdiagram.html diagram] of the interrelations between the derived units based upon the SI units. Definitions of the basic units can be found on this site, as well as the [http://physics.nist.gov/cuu/Constants/codata.pdf CODATA report] listing values for special constants such as the [[electric constant]], the [[magnetic constant]] and the [[speed of light]], all of which have defined values as a result of the definition of the metre and ampere.
<blockquote>In the International System of Units (SI) (BIPM, 2006), the definition of the meter fixes the speed of light in vacuum ''c''<sub>0</sub>, the definition of the ampere fixes the magnetic constant (also called the permeability of vacuum) ''μ''<sub>0</sub>, and the definition of the mole fixes the molar mass of the carbon 12 atom ''M''(<sup>12</sup>C) to have the exact values given in the table [Table 1, p.7]. Since the electric constant (also called the permittivity of vacuum) is related to ''μ''<sub>0</sub> by ''ε''<sub>0</sub> = 1/''μ''<sub>0</sub>''c''<sub>0</sub><sup>2</sup>, it too is known exactly.</blockquote>&nbsp;– CODATA report</ref>

The older metric system included several groups of units. The SI was developed in 1960 from the old [[Mks system of units|metre-kilogram-second]] system, rather than the [[centimetre gram second system of units|centimetre-gram-second]] system, which, in turn, had a few variants. Because the SI is not static, units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses, and as the precision of measurements improves.

The system has been nearly globally [[Metrication|adopted]]. Three principal exceptions are [[Burmese units of measurement|Burma (Myanmar)]], [[Liberia]], and the [[Metrication in the United States|United States]]. The [[Metrication in the United Kingdom|United Kingdom has officially adopted the International System of Units]] but not with the intention of replacing customary measures entirely.

==Realisation of units==
It is very important to distinguish between the definition of a unit and its realisation. The definition of each base unit of the SI is carefully drawn up so that it is unique and provides a sound theoretical basis upon which the most accurate and reproducible measurements can be made. The realisation of the definition of a unit is the procedure by which the definition may be used to establish the value and associated uncertainty of a quantity of the same kind as the unit. A description of how the definitions of some important units are realised in practice is given on the BIPM website.<ref>[http://www.bipm.org/en/si/si_brochure/appendix2/ SI ''Practical Realization'' brochure]</ref>

A coherent SI derived unit can be expressed in SI base units with no numerical factor other than the number&nbsp;1.<ref>Ambler Thompson and Barry N. Taylor, (2008), [http://physics.nist.gov/cuu/pdf/sp811.pdf ''Guide for the Use of the International System of Units (SI)''], (Special publication 811), Gaithersburg, MD: [[National Institute of Standards and Technology]], p.&nbsp;3, footnote&nbsp;2.</ref> The coherent SI derived unit of resistance, the ohm, symbol Ω, for example, is uniquely defined by the relation Ω&nbsp;=&nbsp;m<sup>2</sup>·kg·s<sup>−3</sup>·A<sup>−2</sup>, which follows from the definition of the quantity [[electrical resistance]]. However, "any method consistent with the laws of physics could be used to realise any SI unit."<ref>{{SIbrochure8th|page=111}}</ref> (p.&nbsp;111).

==History==
The [[metric system]] was conceived by a group of scientists (among them, [[Antoine Lavoisier|Antoine-Laurent Lavoisier]], who is known as the "father of modern chemistry") who had been commissioned by [[Louis XVI]] of France to create a unified and rational system of measures. After the [[French Revolution]], the system was adopted by the new government.<ref>{{Cite web|url=http://www1.bipm.org/en/si/history-si/name_kg.html|title=The name "kilogram"|accessdate=25 July 2006}}</ref> On 1 August 1793, the National Convention adopted the new decimal ''metre'' with a provisional length as well as the other decimal units with preliminary definitions and terms. On 7 April 1795 (''Loi du 18 germinal, an III'') the terms ''gramme'' and ''kilogramme'' replaced the former terms ''gravet'' (correctly ''milligrave'') and ''[[grave (mass)|grave]]''. On 10 December 1799 (a month after [[coup of 18 Brumaire|Napoleon's coup d'état]]), the metric system was definitively adopted in France.

[[File:Metrication by year map.svg|thumb|350px|right|Countries by date of metrication]]

The desire for international cooperation on [[metrology]] led to the signing in 1875 of the [[Metre Convention]], a [[treaty]] which established three [[international organization]]s to oversee the keeping of metric standards:
* [[General Conference on Weights and Measures]] (''Conférence générale des poids et mesures'' or CGPM) - a meeting every four to six years of delegates from all member states;
* [[International Bureau of Weights and Measures]] (''Bureau international des poids et mesures'' or BIPM) - an international metrology centre at [[Sèvres]] in France; and
* [[International Committee for Weights and Measures]] (''Comité international des poids et mesures'' or CIPM) - an administrative committee which meets annually at the BIPM.

The [[Metric system#History|history of the metric system]] has seen a number of variations, whose use has spread around the world, to replace many traditional [[Systems of measurement|measurement systems]]. At the end of [[World War II]] a number of different systems of measurement were still in use throughout the world. Some of these systems were metric-system variations, whereas others were based on customary systems. It was recognised that additional steps were needed to [[metrication|promote a worldwide measurement system]]. As a result the 9th [[General Conference on Weights and Measures]] (CGPM), in 1948, asked the [[International Committee for Weights and Measures]] (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.

Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and [[SI electromagnetism units|electromagnetic]] quantities. The six base units that were <!--"that were" expresses the preterit, and the following predicate "are" connotes the non-changing entity no matter where in time of the complements "metre," "kilogram," "second," "ampere," "kelvin," "candela"-->recommended are the [[metre]], [[kilogram]], [[second]], [[ampere]], degree [[Kelvin]] (later renamed the kelvin), and the [[candela]]. In 1960, the 11th CGPM named the system the ''International System of Units'', abbreviated SI from the French name: {{lang|fr|''Le Système international d'unités''}}. The seventh base unit, the [[mole (unit)|mole]], was added in 1971 by the 14th CGPM.

===Future development===
[[ISO 31]] contains recommendations for the use of the International System of Units; for electrical applications, in addition, [[IEC 60027]] has to be taken into account. {{As of|2008}}, work is proceeding to integrate both standards into a joint standard [[ISO/IEC 80000|''Quantities and Units'']] in which the quantities and equations used with SI are to be referred as the ''International System of Quantities'' (ISQ).<ref>[http://www.bipm.org/en/si/si_brochure/chapter1/1-2.html SI Brochure]</ref>

A readable discussion of the present units and standards is found at [http://www.physics.ohio-state.edu/~jossem/IUPAP/PhysicsNowText-A4-1.pdf Brian W. Petley] International Union of Pure and Applied Physics I.U.P.A.P.- 39 (2004).

==Units==
{{Main|SI base unit|SI derived unit|SI prefix}}
The international system of units consists of a set of units together with a set of [[SI prefix|prefixes]]. The units of SI can be divided into two subsets. There are seven [[SI base unit|base units]]: Each of these base units represents, at least in principle, [[Dimensional analysis|different kinds of physical quantities]]<!--"different kinds of physical quantities" is direct from the article "Dimensional_analysis"; "dimensionally independent" is nonsensical and meaningless, serving only to confuse-->. From these seven base units, several [[SI derived unit|other units]] are derived. In addition to the SI units, there is also a set of [[non-SI units accepted for use with SI]] which includes some commonly used units such as the [[litre]].

{| class="wikitable" style="margin:1em auto 1em auto"
|+ SI base units<ref name=sp330>{{Cite book|author=Barry N. Taylor & Ambler Thompson Ed. |title= The International System of Units (SI) |url=http://physics.nist.gov/Pubs/SP330/sp330.pdf |accessdate=18 June 2008|publisher=National Institute of Standards and Technology|location= Gaithersburg, MD|pages=23|year= 2008}}</ref><ref>[http://old.iupac.org/publications/books/author/mills.html Quantities Units and Symbols in Physical Chemistry], IUPAC</ref>
|-
!Name
!Unit symbol
!Quantity
!Symbol
|-
![[metre]]
|'''m'''
|[[length]]
|''l (a lowercase L)''
|-
![[kilogram]] <!--Despite the prefix, kilogram is the base unit because that is the unit that is combined with others to form compound units-->
|'''kg'''
|[[mass]]
|''m''
|-
![[second]]
|'''s'''
|[[time]]
|''t''
|-
![[ampere]]
|'''A'''
|[[electric current]]
|''I (a capital i)''
|-
![[kelvin]]
|'''K'''
|[[thermodynamic temperature]]
|''T''
|-
![[candela]]
|'''cd'''
|[[luminous intensity]]
|''I<sub>v</sub> (a capital i with lowercase v subscript)
|-
![[Mole (unit)|mole]]
|'''mol'''
|[[amount of substance]]
|''n''
|-
|}

A [[SI prefix|prefix]] may be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten. For example, ''kilo-'' denotes a multiple of a thousand and ''milli-'' denotes a multiple of a thousandth; hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a ''milligram'' not a ''microkilogram''.

{{SI-Prefixes}}

==SI writing style==
* Symbols do not have an appended period/full stop (.).
* Symbols are written in upright ([[Roman type|Roman]]) type (m for metres, s for seconds), so as to differentiate from the [[italic type]] used for variables (''m'' for mass, ''s'' for displacement). By consensus of international standards bodies, this rule is applied independent of the font used for surrounding text.<ref name=BIPM2006Ch5>{{Cite journal|author= Bureau International des Poids et Mesures |year=2006 |url=http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |title=The International System of Units (SI) |version= 8th ed. |accessdate=13 February 2008}} Chapter 5.</ref>
* Symbols for units are written in [[lower case]], except for symbols derived from the name of a person. For example, the unit of [[pressure]] is named after [[Blaise Pascal]], so its symbol is written "Pa", whereas the [[Units of measurement|unit]] itself is written "[[pascal (unit)|pascal]]".<ref>Ambler Thompson and Barry N. Taylor, (2008), [http://physics.nist.gov/cuu/pdf/sp811.pdf ''Guide for the Use of the International System of Units (SI)''], (Special publication 811), Gaithersburg, MD: [[National Institute of Standards and Technology]], section 6.1.2 </ref> 
** The one exception is the [[litre]], whose original symbol "l" is unsuitably similar to the numeral "1" or the uppercase letter "i" (depending on the typeface used), at least in many [[List of countries where English is an official language|English-speaking countries]]. The American [[National Institute of Standards and Technology]] recommends that "L" be used instead, a usage which is common in the US, Canada and Australia (but not elsewhere). This has been accepted as an alternative by the [[General Conference on Weights and Measures|CGPM]] since 1979. The cursive ℓ is occasionally seen, especially in Japan and Greece, but this is not currently recommended by any [[Standards organization|standards body]].<!--there are lots of standards organisations and I'm pretty sure at least some of them did recommend this in the past; I think South Africa was also big on this--> For more information, see [[litre]].
* All symbols of prefixes larger than 10<sup>3</sup> (kilo) are uppercase.<ref>Ambler Thompson and Barry N. Taylor, (2008), [http://physics.nist.gov/cuu/pdf/sp811.pdf ''Guide for the Use of the International System of Units (SI)''], (Special publication 811), Gaithersburg, MD: [[National Institute of Standards and Technology]], section 4.3.</ref> 
* The SI rule is that symbols of units are not pluralised, for example "25&nbsp;kg" (not "25&nbsp;kgs").<ref name=BIPM2006Ch5>{{Cite journal|author= Bureau International des Poids et Mesures |year=2006 |url=http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |title=The International System of Units (SI)|version= 8th ed.|accessdate=13 February 2008}} Chapter 5.</ref>
** The American [[National Institute of Standards and Technology]] has defined guidelines for American users of the SI.<ref name=Taylor>{{Cite journal|author=Ambler Thompson & Barry N. Taylor|year=2008|url=http://physics.nist.gov/cuu/pdf/sp811.pdf|title=NIST Special Publication 811: Guide for the Use of the International System of Units (SI)|publisher=National Institute of Standards and Technology|accessdate=18 June 2008}}</ref><ref>{{Cite journal|first=James M. |last=Turner |date=9 May 2008 |url=http://www.gpo.gov/fdsys/pkg/FR-16 May 2008/pdf/E8-11058.pdf |title=Interpretation of the International System of Units (the Metric System of Measurement) for the United States
|publisher=National Archives and Records Administration |journal=Federal Register |volume=73 |issue=96 |pages=28432–3 |id=FR Doc number E8-11058 |accessdate=28 October 2009}}</ref> These guidelines give guidance on pluralising unit names: the plural is formed by using normal [[English grammar]] rules, for example, "henries" is the plural of "[[Henry (unit)|henry]]".<ref name=Taylor/>{{rp|31}} The units [[lux]], [[hertz]], and [[Siemens (unit)|siemens]] are exceptions from this rule: They remain the same in singular and plural. Note that this rule applies only to the full names of units, not to their symbols.
* A space separates the number and the symbol; e.g., "2.21&nbsp;kg", "{{val|7.3|e=2|u=m<sup>2</sup>}}", "22&nbsp;K". This rule explicitly includes the percent sign (%). Exceptions are the symbols for plane angular degrees, minutes and seconds (°,&nbsp;′&nbsp;and&nbsp;″), which are placed immediately after the number with no intervening space.<ref name='BIPM style'>{{Cite book|url= http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |title= The International System of Units (SI) |publisher= International Bureau of Weights and Measures (BIPM) |page=133 |year=2006 |edition=8}}</ref><ref name='nist style'>{{Cite web|url=http://physics.nist.gov/Pubs/SP811/sec07.html |title=NIST Guide to SI Units&nbsp;— Rules and Style Conventions |accessdate=29 December 2009|last1=Thompson|first1=A.|date=July 2008 |last2=Taylor |first2=B. N. |publisher=National Institute of Standards and Technology }}</ref>
* The 10th resolution of [[General Conference on Weights and Measures|CGPM]] in 2003 declared that "the symbol for the [[Decimal separator|decimal marker]] shall be either the [[full stop|point]] on the line or the [[comma]] on the line." In practice, the decimal point is used in English-speaking countries as well as most of Asia and the comma in most continental [[Languages of Europe|European languages]].
* Spaces may be used as a [[thousands separator]] (''{{gaps|1|000|000}}'') in contrast to commas or periods (''1,000,000'' or ''1.000.000'') in order to reduce confusion resulting from the variation between these forms in different countries. [[Space (punctuation)|In print]], the space used for this purpose is typically narrower than that between words (commonly a [[Space (punctuation)#Table of spaces|''thin space'']]).
* Any line-break inside a number, inside a compound unit, or between number and unit should be avoided, but, if necessary, the last-named option should be used.
* Symbols for derived units (formed from multiple units by multiplication) are joined with a [[Interpunct|centre dot]] (·), dot (.),<ref>National Standard of Canada. Canadian Metric Practice Guide, CAN/CSA-Z234.1-89, 1989, item 3.8.1</ref> or a non-break space, for example, "N·m", "N.m", or "N&nbsp;m".<ref>{{Cite book|author=Barry N. Taylor, Ed. |title= The International System of Units (SI) |url=http://physics.nist.gov/Pubs/SP330/sp330.pdf |accessdate=15 October 2007|publisher= National Institute of Standards and Technology|location= Washington, DC|pages=30|year= 2001}}</ref>
* Symbols formed by division of two units are joined with a [[solidus (punctuation)|solidus]] (⁄), or given as a negative [[exponent]]. For example, the "metre per second" can be written "m⁄s", "m&nbsp;s<sup>−1</sup>", "m·s<sup>−1</sup>" or <math>\textstyle\frac{\mathrm{m}}{\mathrm{s}}</math>. Only one [[Solidus (punctuation)|solidus]] should be used; e.g., "kg⁄(m·s<sup>2</sup>)" or "kg·m<sup>−1</sup>·s<sup>−2</sup>" are acceptable but "kg⁄m⁄s<sup>2</sup>" is ambiguous and unacceptable. Many computer users will type the / character provided on [[computer keyboard]]s, which in turn produces the [[Unicode]] character <code>U+002F</code>, which is named solidus but is distinct from the Unicode solidus character, <code>U+2044</code>.
* In [[Chinese language|Chinese]], [[Japanese language|Japanese]], and [[Korean language]] computing ([[CJK]]), some of the commonly used units, prefix-unit combinations, or unit-exponent combinations have been allocated predefined single characters taking up a full square. Unicode includes these in its [http://www.unicode.org/charts/PDF/U3300.pdf CJK Compatibility] and [http://www.unicode.org/charts/PDF/U2100.pdf Letterlike Symbols] subranges for back compatibility, without necessarily recommending future usage.
* When writing dimensionless quantities, the terms 'ppb' (parts per [[Billion (word)|billion]]) and 'ppt' (parts per [[long and short scales|trillion]]) are recognised as language-dependent terms, since the value of billion and trillion can [[Long and short scales|vary from language to language]]. SI, therefore, recommends avoiding these terms.<ref>http://www.bipm.org/en/si/si_brochure/chapter5/5-3-7.html</ref> However, no alternative is suggested by the [[International Bureau of Weights and Measures]] (BIPM).

===Spelling variations===
* The official US spellings for ''deca'', ''metre'', and ''litre'' are ''deka'', ''meter'', and ''liter'', respectively.<ref name='deka'>{{Cite web|url=http://physics.nist.gov/Pubs/SP330/sp330.pdf |title=The International System of Units | pages=iii |accessdate=27 May 2008 }}</ref>
* In some English-speaking countries, the unit ''ampere'' is often shortened to ''amp'' (singular) or ''amps'' (plural) in informal writing as well as on many electrical appliances. ''Secs'' may sometimes be seen instead of ''s'' or ''seconds''.

==Conversion factors==
The relationship between the units used in different systems is determined by convention or from the basic definition of the units. Conversion of units from one system to another is accomplished by use of a [[Conversion of units|conversion factor]]. There are several compilations of conversion factors; see, for example, Appendix B of NIST SP 811.<ref name=Taylor/>

==Length, mass, and temperature convergence==
Density (specific mass) is commonly expressed in SI units or in reference to water. Since a cube with sides of 1 decimetre has volume of 1 cubic decimetre</sup>, which is 1 [[litre]] and, when filled with water, has an approximate mass of 1 kilogram, water has an approximate density of 1 kilogram per litre, which is equal to 1&nbsp;gram per cubic centimetre</sup> and 1 tonne per cubic metre, and will freeze at approximately 0 degrees Celsius at 1 atmosphere of pressure.

Note that this is only an approximate definition of the [[kilogram]], as the density of water can change with temperature; the actual definition is based on a specific platinum-iridium cylinder held in a vault at the [[BIPM]] in Sèvres, France.

==Cultural issues==
[[Image:Non-Metric User.svg|thumb|right|upright=1.5|Three nations have not officially adopted the International System of Units as their primary or sole system of measurement: [[Burmese units of measurement|Burma]], [[Liberia#Weights_and_measures|Liberia]], and the [[United States customary units|United States]].]]
The near-worldwide adoption of the metric system as a tool of economy and everyday commerce was based to some extent on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardise the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. For use in science, it simplifies dealing with very large and small quantities, since it lines up so well with the [[decimal]] [[numeral system]].

Many units in everyday and scientific use are not derived from the seven SI base units (metre, kilogram, second, ampere, kelvin, mole, and candela) combined with the SI prefixes. In some cases these deviations have been approved by the BIPM.<ref>[http://www.bipm.org/en/si/si_brochure/chapter4/table8.html BIPM - Table 8<!-- Bot generated title -->]</ref> Some examples include:
* The many units of time ([[minute]], min; [[hour]], h; [[day]], d<!--- the only three in table 6 --->) in use besides the SI second, and are specifically accepted for use according to table 6.<ref>[http://www.bipm.org/en/si/si_brochure/chapter4/table6.html BIPM - Table 6<!-- Bot generated title -->]</ref>
* The year is specifically not included but has a recommended conversion factor.<ref>[http://physics.nist.gov/Pubs/SP811/appenB9.html#TIME NIST Guide to SI Units - Appendix B9. Conversion Factors<!-- Bot generated title -->]</ref>
* The [[Celsius]] temperature scale; kelvins are rarely employed in everyday use.
* [[Electrical energy|Electric energy]] is often billed in [[watt-hour|kilowatt-hours]] instead of megajoules. Similarly, battery charge is often measured as miliamperes-hour (mAh) instead of [[coulomb]]s.
* The [[nautical mile]] and [[knot (speed)|knot]] (nautical mile per hour) used to measure travel distance and speed of ships and aircraft (1 International nautical mile = {{gaps|1|852}}&nbsp;m or approximately 1 minute of latitude). In addition to these, Annex 5 of the [[Convention on International Civil Aviation]] permits the "temporary use" of the [[foot (length)|foot]] for [[altitude]].
* [[Astronomical distance]]s measured in [[astronomical unit]]s, [[parsec]]s, and [[light-year]]s instead of, for example, petametres (a light-year is about 9.461&nbsp;Pm or about {{gaps|9|461|000|000|000|000}}&nbsp;m).
* Atomic scale units used in physics and chemistry, such as the [[ångström]], [[electron volt]], [[atomic mass unit]] and [[barn (unit)|barn]].
* Some [[physicist]]s prefer the [[centimetre gram second system of units|centimetre-gram-second]] (CGS) units, or systems based on [[physical constant]]s, such as [[Planck units]], [[atomic units]], or [[Geometrized unit system|geometric units]].<!-- these are natural units which are not similar to cgs units -->
* In some countries, the informal [[cup (unit)|cup]] measurement has become 250&nbsp;mL. Likewise, a 500&nbsp;g [[Pound (mass)#Metric pounds|metric pound]] is used in many countries. Liquids, especially alcoholic ones, are often sold in units whose origins are historical (for example, [[pint]]s for beer and cider in glasses in the UK&nbsp;—although ''pint'' means 568&nbsp;mL; champagne in [[Wine bottle#Sizes|Jeroboams]] in France).
* A [[Mile#Other miles|metric mile]] of 10&nbsp;km is used in Norway and Sweden. The term metric mile is also used in some English speaking countries for the 1500&nbsp;m foot race.
* In the US, [[Blood sugar|blood glucose]] measurements are recorded in milligrams per decilitre (mg/dL), which would normalise to cg/L; in Canada, Australia, New Zealand, Oceania, and Europe, the standard is millimole per litre (mmol/L) or mM (millimolar).
* [[Blood pressure]] and [[atmospheric pressure]] are usually measured in [[mmHg]] and [[Bar (unit)|bar]]s, respectively, instead of Pa.

The fine-tuning that has happened to the metric base-unit definitions over the past 200 years, as experts have tried periodically to find more precise and reproducible methods, does not affect the everyday use of metric units. Since most non-SI units in common use, such as the [[US customary units]], are nowadays defined in SI units,<ref>Mendenhall, T. C. (1893). "Fundamental Standards of Length and Mass". Reprinted in Barbrow, Louis E. and Judson, Lewis V. (1976). ''Weights and measures standards of the United States: A brief history (NBS Special Publication 447).'' Washington D.C.: Superintendent of Documents. Viewed 23 August 2006 at [http://physics.nist.gov/Pubs/SP447/ http://physics.nist.gov/Pubs/SP447/] pp. 28–29.
</ref> any change in the definition of the SI units results in a change of the definition of the older units, as well.

===International Trade===
One of the [[European Union|European Union's]] (EU) objectives is the creation of a single market for trade. In order to achieve this objective, the EU standardised on using SI as the legal units of measure.  At the time of writing (2009) it had issued two [[Unit of measurement|units of measurement]]  [[Directive (European Union)|directives]] which catalogued the units of measure that might be used for, amongst other things, trade: the first was Directive 71/354/EEC<ref>{{Cite web| url = http://eur-lex.europa.eu/Notice.do?mode=dbl&lang=en&lng1=en,nl&lng2=da,de,el,en,es,fr,it,nl,pt,&val=22924:cs&page=1&hwords= | title=Council Directive of 18 October 1971 on the approximation of laws of the member states relating to units of measurement, (71/354/EEC) | accessdate=7 February 2009}}</ref> issued
in 1971 which required member states to standardise on SI rather than use the variety of  [[Centimetre gram second system of units|cgs]] and  [[Mks system of units|mks]] units then in use.  The second was Directive 80/181/EEC<ref>{{Cite web| url = http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:19791221:EN:PDF | author = The Council of the European Communities | title = Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC | date=21 December 1979 | accessdate=7 February 2009}}</ref><ref>{{Cite web| url = http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:19841220:EN:PDF | author = The Council of the European Communities | title = Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC | date=20 December 1984 | accessdate=7 February 2009}}</ref><ref>{{Cite web| url = http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:19891130:EN:PDF | author = The Council of the European Communities | title = Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC | date=30 November 1989 | accessdate=7 February 2009}}</ref><ref>{{Cite web| url = http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:20000209:EN:PDF | author = The Council of the European Communities | title = Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC | date=9 February 2000 | accessdate=7 February 2009}}</ref><ref>{{Cite web| url = http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:20090527:EN:PDF| author = The Council of the European Communities | title = Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC | date=27 May 2009 | accessdate=14 September 2009}}</ref> issued in 1979 which replaced the first and which gave the [[United Kingdom]] and the [[Republic of Ireland]] a number of [[derogation]]s from the original directive.

The directives gave a derogation from using SI units in areas where other units of measure had either been agreed by international treaty or which were in universal use in worldwide trade.  They also permitted the use of ''supplementary indicators'' alongside, but not in place of the units catalogued in the directive.  In its original form, Directive 80/181/EEC had a cut-off date for the use of such indicators, but with each amendment this date was moved until, in 2009, ''supplementary indicators'' have been allowed indefinitely.

==See also==
{|
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* [[SI derived unit]]
* [[Dimensional analysis]]
* [[History of measurement]]
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* [[List of international common standards]]
* [[Orders of magnitude]]
* [[Long and short scales]]
|-
|'''Organisations'''
|-valign=top
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* [[Institute for Reference Materials and Measurements]] (IRMM)
| width=40 |
|
* [[Committee on Data for Science and Technology|CODATA]]
|-
|'''Standards and conventions'''
|-valign=top
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* [[Coordinated Universal Time]] (UTC)
|
|
* [[ISO 1000]]
| width=40 |
|}
{{systems of measurement}}

==References==
{{Reflist|2}}

==Further reading==
* {{GreenBookRef}}
* [http://info.ee.surrey.ac.uk/Workshop/advice/coils/unit_systems/#rms Unit Systems in Electromagnetism]
* [http://qdev.boulder.nist.gov/817.03/pubs/downloads/set/Watt_Triangle_sub1.pdf MW Keller ''et al.''] Metrology Triangle Using a Watt Balance, a Calculable Capacitor, and a Single-Electron Tunneling Device

==External links==
;Official
* [http://www.bipm.org/en/si/ BIPM Bureau International des Poids et Mesures (SI maintenance agency)] (home page)
** [http://www.bipm.org/en/si/si_brochure/ BIPM brochure] (SI reference)
* [http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=5448&ICS1=1 ISO 1000:1992 ''SI units and recommendations for the use of their multiples and of certain other units'']
** [http://www.iso.org/iso/en/CatalogueListPage.CatalogueList?COMMID=542&scopelist=ALL ISO 31/1000/80000]
* [http://physics.nist.gov/cuu/Units/bibliography.html NIST Official Publications]
** [http://physics.nist.gov/Pubs/SP330/sp330.pdf NIST Special Publication 330, 2008 Edition: The International System of Units (SI)]
** [http://ts.nist.gov/WeightsAndMeasures/Metric/pub814.cfm NIST Special Pub 814: Interpretation of the SI for the United States and Federal Government Metric Conversion Policy]
* [http://laws.justice.gc.ca/en/ShowTdm/cs/W-6///en Weights and Measures Act, Canada]
* [http://webstore.ansi.org/ansidocstore/product.asp?sku=SI10%2D2002 IEEE/ASTM SI 10-2002 Standard for Use of the International System of Units (SI): The Modern Metric System] (ANSI approved, joint IEEE/ASTM standard)
* [http://www.sae.org/standardsdev/tsb/tsb003.pdf Rules for SAE Use of SI (Metric) Units]
* [http://www.npl.co.uk/server.php?show=category.364 National Physical Laboratory, UK]
;Information
* {{dmoz|Science/Reference/Units_of_Measurement/}}
* [http://www.engnetglobal.com/tips/convert.aspx EngNet Metric Conversion Chart] Online Categorised Metric Conversion Calculator
* [http://lamar.colostate.edu/~hillger/pdf/Practical_Guide_to_the_SI.pdf A Practical Guide to the International System of Units]

;History
* [ftp://cam.ctan.org/tex-archive/macros/latex/contrib/SIunits/SIunits.pdf LaTeX SIunits package manual] gives a historical background to the SI system.

;Research
* [http://www.npl.co.uk/server.php?show=ConWebDoc.1835 ''The metrological triangle'']
* [http://www.bipm.org/cc/CIPM/Allowed/94/CIPM-Recom1CI-2005-EN.pdf Recommendation of ICWM 1 (CI-2005)]
;Pro-metric [[advocacy groups]]
* [http://www.ukma.org.uk/ The UK Metric Association]
* [http://www.metric.org/ The US Metric Association]
* [http://niagara.cioc.ca/details.asp?RSN=5108&Number=0 Canadian Metric Association]
* [http://www.metrication.us Metrication US]

;Pro-customary measures pressure groups
* {{dmoz|Society/Issues/Government_Operations/Anti-Metrication/|Pro-customary measures groups}}

{{SI units navbox}}

{{Use dmy dates|date=September 2010}}

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