nonius
A device for adjusting the accuracy of mathematical instruments, later superseded by the vernier.
Well, of course, one must manipulate the various Screw-Settings precisely, read the Nonius, and an hundred other details besides I’d but bore you with,—
Where it comes from
Borrowed from Latin Nonius, the Latinised form of the name of its inventor, Pedro Nunes (1502-78), a Portuguese mathematician and geographer.
Adapted from the Wiktionary entry nonius (CC BY-SA 4.0).