Why the World Agreed on A4 Paper

The A series rests on a single elegant rule, every sheet has the same proportions as the next, so folding A4 in half produces A5. Georg Christoph Lichtenberg described the property in 1786, Germany standardized it as DIN 476 in 1922, and ISO 216 carried it to the world in 1975.

An A0 sheet is defined as one square meter, letting papermakers compute weight per sheet exactly. Only the United States and a few neighbors still print on Letter, an island of 8.

5 by 11 inch paper in an A4 world.