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metre

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The metre is the length of the path travelled by light in vacuum during a time interval of ${1/299\:792\:458}$ of a second.

This definition is valid from 1983-10-17 to 2019-05-19
Unit metre
Symbol m
Quantity length
Defining Constant speed of light
Defining Resolution
CGPM Resolution 1 (1983)
Unit Type SI base unit
Notes:
  1. The old international prototype of the metre which was sanctioned by the 1st CGPM in 1889 is still kept at the BIPM under the conditions specified in 1889.
www.bipm.org