Article
Paper sizes A0 to A8, life-size installation The Invasion of the Square Roots at the CosmoCaixa Barcelona science museum A size chart illustrating the ISO A series and a comparison with American letter and legal formats Comparison of some paper and photographic paper sizes close to the A4 size
Paper size standards govern the size of sheets of paper used as writing paper, stationery, cards, and for some printed documents. Most countries adhere to the ISO 216 standard, which includes the widely recognized A series (including A4 paper), defined by a consistent aspect ratio of √2. The system, first proposed in the 18th century and formalized in 1975, allows scaling between sizes without distortion. Regional variations exist, such as the North American paper sizes (e.g., Letter, Legal, and Ledger) which are governed by the ANSI and are used in North America and parts of Central and South America.