I can't believe I might have to explain what this is, it's sort of like explaining the pencil, but apparently in modern printing production the proportion wheel is nowhere to be found. We used this to figure out what percentage to reduce or enlarge images in the preparation of negatives, and to figure out cropping to a set amount of space.
Apparently C-Thru still sells these.
My wife is currently instructing 2d-year college design students on how to use a T-square. I mean, really.