ISO 216 specifies
international standard (ISO)
paper sizes used in most countries in the world today. It is the standard which defines the commonly available
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]
Paper in the A series format has a 
aspect ratio, although this is rounded to the nearest millimetre. A0 is defined so that it has an area of 1 m², prior to the above mentioned rounding. Successive paper sizes in the series (A1, A2, A3, etc.) are defined by halving the preceding paper size, cutting parallel to its shorter side (so that the long side of An+1 is the same length as the short side of An, again prior to rounding).
The most frequently used of this series is the size A4 (210 × 297 mm). A4 paper is 6 mm narrower and 18 mm longer than the "Letter" paper size, 8½ × 11 inches (216 × 279 mm), commonly used in North America.
