I’m an experimental physicist who’s spent 30 years calibrating scientific instruments. In 2012, I invented a technique called ‘self-calibration’ of the polarization angle of Cosmic Microwave Background telescopes that’s still used to this day. Recently I discovered that the word “calibrate” may itself need calibration. It comes from “caliber,” via French calibre. After that, the trail splits. The traditional derivation traces it to Arabic qālib, “mold” or “form,” possibly through Syriac to a Greek word for a shoemaker’s last, the foot-shaped form used to make a shoe. Which is wonderfully appropriate. Long before SI units, human bodies supplied our standards of length. The foot. The cubit, or amah, based on the forearm of the Pharaoh. But there is a rival etymology. Calibre may instead come from Medieval Latin qua libra, “of what weight?”, from libra, the balance or scales. Look again: ca-LIBRA-te. After three decades in experimental physics, I find it slightly alarming that even the word for checking a measurement against a standard comes with an error budget. cc: @MerriamWebster