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

I had a similar thought about straight edges. How did the first perfectly straight line get created? How did they know it was straight if there was nothing to compare it to… etc etc

Answer 2

I had a similar thought about straight edges. How did the first perfectly straight line get created? How did they know it was straight if there was nothing to compare it to… etc etc