Is there like a master ruler? Who decided what a length a cm, mm and inch would be and why?
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Much of the world adheres to OIML recommendations for measurement.
Length measures are largely addressed by OIML-R35
https://www.oiml.org/en/files/pdf_r/r035-1-e07.pdf/@@download/file/R035-1-e07.pdf
Edit: yes, there are master standards (measures) for most types of measurements (length, mass, volume, etc.). These are used for intermediate verifications and are often referred to as transfer standards. The actual measurement realization is usually from a scientific process that is repeatable. These are considered the ‘base units’. For example, the definition for length was changed from a physical standard (prototype of the metre) to a certain number of wavelengths of a spectral line of a krypton-86 radiation, making it derivable from universal natural phenomena. Volume as mentioned previously is actually a measurement derived from length.
https://en.m.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units
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