Every A-series sheet is √2 times as tall as it is wide, so halving one gives the next size down in exactly the same shape — and A0 is one square metre, so a sheet of ordinary 80 g/m² A4 weighs 5 g, and postage is arithmetic. Markus Kuhn’s page traces the idea to a 1786 letter in which the physicist Georg Christoph Lichtenberg found his writing paper already had the ratio, and explains why an A4 letter slides into a C4 envelope that slides into a B4.