ISO 216 defines the most common paper size standard in operation in the world today. With the exception of the United States, Canada, Peru, Mexico, Colombia and The Dominican Republic, the ISO 216 is used in every country to specify standard “A” size and “B” size paper, including the most common sized paper in the world, A4.

Alongside ISO 216, there exists two other ISO standards, ISO 217 and ISO 269, that together define related paper sizes. The ISO 269 defines the “C” sized paper series, which is closely related to both “A” and “B” sized paper.

With the exception of some envelope sizes, ISO 216, ISO 217 and ISO 269 share the same aspect ratio of √2:1, within rounding to millimeters. This has the unique property that folding the paper in half will produce the same sized aspect ratio for the two halves of paper. This makes sizing easier to maintain and eases use for developers and paper users.