If you ever wondered what the hell the “A4” paper size was, and needed to go beyond the “it’s about the same at ‘US letter'” level, then prepare to have your noodle baked. Because not only is A4 metric, it’s recursive, exponential and proportional! Jesus, no wonder America is the dumbest place on the planet. We test for this crap–everyone else just assumes you know it. And the Bn and Cn formats? Let me put it this way, if you didn’t take A-levels, you don’t want to know.
The width and height of a Bn format are the geometric mean between those of the An and the next larger A(n-1) format…. Similarly, the formats of the C series are the geometric mean between the A and B series formats with the same number.
See what I mean? I’d move, but I’d have to go back to grade school.