Is there like a master ruler? Who decided what a length a cm, mm and inch would be and why?
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So, this is fun. I’m going to talk about mass instead of length, because its what I have experience in, but the process was/is very similar.
About 15 years ago i worked in a laboratory that did environmental testing. We would get samples in from construction or clean-up sites and test for specific contaminates (heavy metals, gas, diesel, etc). In the lab we had a set of weights that we would use to calibrate our scales every day, and those weights were traceable back to the “master” weights.
every year, our weights would get sent out to a calibration lab. That lab would verify our weights were still accurate. They did that by comparing them to a set of weights they kept for the purpose, which in turn were sent out every year to be compared to another set, and to another set. This would eventually lead back to the Master Weight.
All of those sets had paperwork that would trace back to the master weight, so the set was said to be “traceable”
As others have said, nowadays weights are not defined by a physical sample, but by a mathematical standard. Here is an article from 2018, when it was decided that the definition of the Kg would change, which offically happened in 2019.
[New kilogram standard: how the SI unit of mass is being redefined – Vox](https://www.vox.com/science-and-health/2018/11/14/18072368/kilogram-kibble-redefine-weight-science)
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