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ISO 216 specifies international standard (ISO) paper sizes used in most countries in the world today. It is the standard which defines the commonly available A4 paper size. The underlying principle is that when folded in half length-wise the paper retains its original aspect ratio: 1:√2

I think 1:√2 is a lovely ratio, don’t you? Now someone please tell me why we don’t use the ISO 216 system in America. Go on, then.