Comment by rswail

Not only that but C envelope sizes match the A size. So an A4 piece of paper fits a C4 envelope flat.

A4 folded in half (size of an A5) fits in a C5 envelope.

An ISO standard that makes sense and isn't based on different professions like "letter" vs "legal" vs "folio" and other US sizes.

But also the reason that, for example, screens have 80 columns, (also related to punch cards), but that was about the width of a "letter" page at 10cpi.