The ISO 216 standard defines the A‑ and B‑series paper sizes used throughout most of the world. Both series share the same aspect ratio, which allows each sheet to be halved while preserving its proportions. The A‑series is anchored by A0, a sheet with an area of exactly 1 square meter, with each subsequent size created by halving the previous one. A4 (210 x 297 mm) is the most common everyday format, similar to US letter paper. The B‑series fills the gaps between A‑sizes, with each B‑size being the geometric mean of the adjacent A‑sizes and B0 defined as 1 meter wide.