Gauge blocks, a system for producing precision lengths

If you're interested in precision machining, Robrenz is a good channel to check out:

https://www.youtube.com/user/ROBRENZ/videos

He demos a Brown & Sharpe electronic indicator here, showing how sensitive they are, even to errant breaths: https://youtu.be/UG6LV8v8W-0?t=25m15s

AvE did a few vids on the "wringing" phenomena.

https://www.youtube.com/watch?v=gbsd2OpPOMw

Having now read the whole Wikipedia article I can say it was way more interesting than I expected at first.