[aastronaut]

Had those in heavy use during my work on a surface grinder[0], I can remember it had an ancient digital interface with 0.0001mm precision. We used the blocks to calibrate our measuring table[1] constantly. Later in life I got to see the machines to measure the quality of photomasks[2] in the production line of modern CPUs, and I was just blown away when I heard about those tolerances.

[0]: https://en.wikipedia.org/wiki/Surface_grinding

[1]: https://www.bafasan.com/en/images/kke/kke2.png

[2]: https://www.zeiss.com/semiconductor-manufacturing-technology...