ISO Paper, Envelope and Poster sizes explained

ISO specifies the international paper sizes used in most countries in the world today. ISO 216 defines the 'A' series of sizes which is used for paper, and the 'B' series of sizes which are historically poster sizes.

Paper ('A') sizes have an aspect ratio of 1:square root of 2. I know, I know.... but what that means is that when an 'A' size sheet of paper is torn or folded in half across the long side it always maintains its aspect ratio.

ISO 269 specifies the 'C' series of sizes which is used for envelopes, and complements the 'A' series of paper sizes