In my letterpress classes I am constantly reminding students to use a ruler or pica pole to measure things — Is the type straight on the page? Is it really centered? How much furniture do they need for the lockup? Some resist (although everyone comes around by the end of the day), but others start immediately to reminisce about using a type gauge, like the Haberule ones on the left.
I looked up the Haberule the other day, to see if perhaps I could find one to show students. I immediately found the Museum of Forgotten Art Supplies, full of tools that were useful not-so-long-ago, but have been mostly supplanted by computers. Look here for info on the Haberule (be sure to look at the comments, which explain how it works).