The first definition, used between 1889 and 1960, was by the international prototype meter.

Mass, energy, and force

Following the 2019 revision of the SI, electromagnetic radiation, in general, was explicitly defined to have the exact parameters:
* c = 299,792,458 m/s
* h = J s

The caesium-133 hyperfine transition radiation was explicitly defined to have frequency:
* = 9,192,631,770 Hz

Though

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