Units of Measurement/Length

SI units
The base unit of length in the SI system is the metre. This has had several definitions over time:


 * 1889
 * The International Bureau of Weights and Measures defines the metre as the distance between two marks on a platinum-iridium alloy bar (the International Prototype Metre). This prototype is kept in a vault in Sèvres, near Paris.


 * 1960
 * The 11th General Conference on Weights and Measures redefines the metre in terms of a specific number of wavelengths of light of a specific frequency in a vacuum. It is defined to be 1&thinsp;650&thinsp;763.73 wavelengths of the electromagnetic radiation emitted by an electron transitioning between the levels 2p10 and 5d5 of the krypton-86 (86Kr) atom in a vacuum. The metre is now defined in terms of reproducible physical effects, rather than a synthetic artifact.
 * 1983
 * The 17th General Conference on Weights and Measures redefines the metre in terms of the distance traversed by light (having a universally constant speed in a vacuum) in a specific time. It is defined as the distance traveled by a beam of light in a vacuum in 1/299&thinsp;792&thinsp;458 second. This is the current definition. This has the effect that the speed of light is defined to be exactly 299&thinsp;792&thinsp;458 m/s.

Imperial units
One international yard is defined as 0.914&thinsp;4 m