ELI5, How are precision calibration tools, themselves calibrated?

Question

Feels like a chicken and egg senario. Let’s say I get my torque wrench from work sent off to be calibrated, and that’s calibrated with something itself needs to be calibrated, and so on and so fourth. How’s that figured out?

In: 430

Answer 1

Also see how to handscrape plane surfaces. There are tricks where you can craft something completely flat, starting from imperfect parts. Tricks like those are used to ‘pull up by your own hair’, which is what toolmakers have to do.
That specific trick involves scraping 3 surfaces against each other, in a way that eventually makes all 3 perfectly flat.

Answer 2

Also see how to handscrape plane surfaces. There are tricks where you can craft something completely flat, starting from imperfect parts. Tricks like those are used to ‘pull up by your own hair’, which is what toolmakers have to do.
That specific trick involves scraping 3 surfaces against each other, in a way that eventually makes all 3 perfectly flat.