A5 paper size

ISO 216 specifies international standard (ISO) paper sizes used in most countries in the world today. It is the standard which defines the well-known A4 paper size.The international ISO standard is based on the German DIN standard 476 (DIN 476) from 1922. Some of the formats contained therein were independently invented in France during its revolution and later forgotten. [1] The aspect ratio used by this norm was mentioned in a letter by the German Georg Christoph Lichtenberg written 1786-10-25. [2]
See more at Wikipedia.org...