The A-series consists of a logical set of paper sizes that are defined by the ISO 216 standard. The largest size (A0) measures one square meter. The height-width ratio is constant (1 : 1.41) for all sizes.

You get the A1 size by folding an A0 paper in two along its shortest side. Then, fold the A1 size in two to get an A2 size paper, and so on.