spoiler
For people that don’t know this is not how you use Calipers
That’s what the two prongs at the top are for. Flip the caliper upside down, use the prongs to measure the inside dimension, and read it off the same scale.
Ah cool, didn’t notice those. Cheers
“Caliper jaws for inside measurement—I thought of that. Turned out it already existed, but I arrived at it independently."
I think the guy was actually referring to something a bit different, that is having a second number scale on the caliper that is offset by the width of the first jaw, so you can use the outside jaws for measuring inside dimensions. I don’t think that would work, however.
The second scale sounds like a good Idea till you mess up everything due to using the wrong one. I once had a Spirit Level that was for plumbers and had a Second bubble-level built in that was even when the Level was tiltet to about 1.5 degree, great for waste-lines and gutters. Now everything in my House ist tilted by 1.5 degree except the plumbing and gutters.
By using the same scale for inside and outside, you can take one measurement inside, and compare it to something else as outside without moving the scale at all.