Each is half the area of the one before, ie A4 paper (297 mm × 210 mm) is half the size of A3 paper (297 mm × 420 mm).

Developed in the 1940s, it was a concept based on the A3 sheet of paper, which is 11 by 17 inches in size, at the time, the biggest size of paper used in a photocopying machine. The idea was that ...