The metre (Commonwealth English ) or meter (American English ) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light  in absolute vacuum during a time  interval of 1/299,792,458 of a second .
SI prefixes applied to the metre
Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo- = 1000) and millimetre (one thousandth of a metre; milli- = 1 / 1 000).
The metre may be used with SI prefixes as shown.
1 metre is equivalent to:
- exactly 1/0.9144 yards  (approximately 1.0936 yards)
- exactly 1/0.3048 feet (approximately 3.2808 feet)
- exactly 10000/254 inches  (approximately 39.370 inches)
See History at 
Timeline of definition
- May 8, 1790 — The National Assembly (French Revolution)  decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second.
- March 30, 1791 — The French National Assembly accepts the proposal by the French Academy of Sciences  that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian (geography) along a quadrant (one-fourth the polar circumference of the earth).
- 1795 — Provisional metre bar constructed of brass .
- December 10, 1799 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.
- September 28, 1889 — The first Conférence Générale des Poids et Mesures  defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium , measured at the melting point of ice.
- October 6, 1927 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmospheric pressure  and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
- October 20, 1960 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths  in vacuum of the electromagnetic radiation  corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom.
- October 21, 1983 — The seventeenth CGPM defines the length to be distance travelled by light  in vacuum during a time interval of 1/299 792 458 of a second .
- Metric system
- SI prefix
- Conversion of units for comparisons with other units
- Orders of magnitude (length)
- Speed of light
- History of the metre at the U.S. National Institute of Standards and Technology (NIST)
- Timeline of history of the metre at the NIST
- Bureau International des Poids et Measures - Lengths
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