Its mathematically derived. A0 is exactly 1m2 in area. The side lengths are at a ratio of sqrt(2), that means you can cut any A paper size in half and get the s

Its mathematically derived. A0 is exactly 1m2 in area. The side lengths are at a ratio of sqrt(2), that means you can cut any A paper size in half and get the size below it. B series are the same ratio, but B0 has a width of 1m instead.