# Metre

The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second, where the second is defined by a hyperfine transition frequency of caesium.
The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's polar circumference is approximately 40000 km.
In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in vacuum in ⁠1/299792458⁠ of a second. After the 2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequency Δν$_{Cs}$. This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures 40007.863 km, a change of about 200 parts per million from the original value of exactly 40000 km, which also includes improvements in the accuracy of measuring the circumference.

## Spelling

Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations, the exceptions being the United States and the Philippines which use meter.
Measuring devices (such as ammeter, speedometer) are spelled "-meter" in all variants of English. The suffix "-meter" has the same Greek origin as the unit of length.

## Etymology

The etymological roots of metre can be traced to the Greek verb μετρέω (metreo) ((I) measure, count or compare) and noun μέτρον (metron) (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (metior, mensura), French (mètre, mesure), English and other languages. The Greek word is derived from the Proto-Indo-European root *meh₁- 'to measure'. The motto ΜΕΤΡΩ ΧΡΩ (metro chro) in the seal of the International Bureau of Weights and Measures (BIPM), was approuved by Adolphe Hirsch on 11 July 1875 and may be translated as "Keep the measure", thus calls for both measurement and moderation. The use of the word metre (for the French unit mètre) in English began at least as early as 1797.

## History of definition

During French Revolution, the the traditional units of measure were to be replaced by consistent measures based on natural phenomena. As a base unit of length, scientists had favoured the seconds pendulum (a pendulum with a half-period of one second) one century earlier, but this was rejected as it had been discovered that this length varied from place to place with local gravity. A new unit of length, the metre was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth flattening of ⁠1/334⁠.
The historical French official standard of the metre was made available in the form of the Mètre des Archives, a platinum bar held in Paris. During the mid nineteenth century, following the American Revolution and independence of Latin America, the metre gained adoption in Americas, particularly in scientific usage, and it was officially established as an international measurement unit by the Metre Convention of 1875 at the beginning of the Second Industrial Revolution.
The Mètre des Archives and its copies such as the Committee Meter were replaced from 1889 at the initiative of the International Geodetic Association by thirty platinum-iridium bars kept across the globe. A better standardisation of the new prototypes of the metre and their comparison with each other and with the historical standard involved the development of specialised measuring equipment and the definition of a reproducible temperature scale.
Progress in science finally allowed the definition of the metre to be dematerialised; thus in 1960 a new definition based on a specific number of wavelengths of light from a specific transition in krypton-86 allowed the standard to be universally available by measurement. In 1983 this was updated to a length defined in terms of the speed of light; this definition was reworded in 2019:
   The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s$^{−1}$, where the second is defined in terms of the caesium frequency Δν$_{Cs}$.

## Early adoptions of the metre internationally

In France, the metre was adopted as an exclusive measure in 1801 under the Consulate. This continued under the First French Empire until 1812, when Napoleon decreed the introduction of the non-decimal mesures usuelles, which remained in use in France up to 1840 in the reign of Louis Philippe. Meanwhile, the metre was adopted by the Republic of Geneva. After the joining of the canton of Geneva to Switzerland in 1815, Guillaume Henri Dufour published the first official Swiss map, for which the metre was adopted as the unit of length.

### Adoption dates by country

* France: 1801–1812, then 1840
* Republic of Geneva, Switzerland: 1813
* Kingdom of the Netherlands: 1820
* Kingdom of Belgium: 1830
* Chile: 1848
* Kingdom of Sardinia, Italy: 1850
* Spain: 1852
* Portugal: 1852
* Colombia: 1853
* Ecuador: 1856
* Mexico: 1857
* Brazil: 1862
* Argentina: 1863
* Italy: 1863
* United States: 1866
* German Empire, Germany: 1872
* Austria, 1875
* Switzerland: 1877

## SI prefixed forms of metre

SI prefixes can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km, astronomical units (149.6 Gm), light-years (10 Pm), or parsecs (31 Pm), rather than in Mm or larger multiples; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.
The terms micron and millimicron have been used instead of micrometre (μm) and nanometre (nm), respectively, but this practice is discouraged.
SI multiples of metre (m) :
| Submultiples | Submultiples | Submultiples | Multiples | Multiples | Multiples |
| Value | SI symbol | Name | Value | SI symbol | Name |
| 10$^{−1}$ m | dm | decimetre | 10$^1$ m | dam | decametre |
| 10$^{−2}$ m | cm | centimetre | 10$^2$ m | hm | hectometre |
| 10$^{−3}$ m | mm | millimetre | 10$^3$ m | km | kilometre |
| 10$^{−6}$ m | μm | micrometre | 10$^6$ m | Mm | megametre |
| 10$^{−9}$ m | nm | nanometre | 10$^9$ m | Gm | gigametre |
| 10$^{−12}$ m | pm | picometre | 10$^{12}$ m | Tm | terametre |
| 10$^{−15}$ m | fm | femtometre | 10$^{15}$ m | Pm | petametre |
| 10$^{−18}$ m | am | attometre | 10$^{18}$ m | Em | exametre |
| 10$^{−21}$ m | zm | zeptometre | 10$^{21}$ m | Zm | zettametre |
| 10$^{−24}$ m | ym | yoctometre | 10$^{24}$ m | Ym | yottametre |
| 10$^{−27}$ m | rm | rontometre | 10$^{27}$ m | Rm | ronnametre |
| 10$^{−30}$ m | qm | quectometre | 10$^{30}$ m | Qm | quettametre |

## Equivalents in other units

| Metric unit expressed in non-SI units | Metric unit expressed in non-SI units | Metric unit expressed in non-SI units | Metric unit expressed in non-SI units | Non-SI unit expressed in metric units | Non-SI unit expressed in metric units | Non-SI unit expressed in metric units | Non-SI unit expressed in metric units |
| 1 metre | ≈ | 1.0936 | yard | 1 yard | = | 0.9144 | metre |
| 1 metre | ≈ | 39.370 | inches | 1 inch | = | 0.0254 | metre |
| 1 centimetre | ≈ | 0.39370 | inch | 1 inch | = | 2.54 | centimetres |
| 1 millimetre | ≈ | 0.039370 | inch | 1 inch | = | 25.4 | millimetres |
| 1 metre | = | 10$^{10}$ | ångström | 1 ångström | = | 10$^{−10}$ | metre |
| 1 nanometre | = | 10 | ångström | 1 ångström | = | 100 | picometres |
Within this table, "inch" and "yard" mean "international inch" and "international yard" respectively, though approximate conversions in the left column hold for both international and survey units.
    "≈" means "is approximately equal to";
    "=" means "is exactly equal to".
One metre is exactly equivalent to ⁠5 000/127⁠ inches and to ⁠1 250/1 143⁠ yards.
A simple mnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3 feet 3+3⁄8 inches. This gives an overestimate of 0.125 mm.
The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm). Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly 1⁄2 toise. The Russian verst was 1.0668 km. The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.