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Many paper size standards conventions have existed at different times and in different countries. Today, the A and B series of ISO 216, which includes the commonly used A4 size, are the international standard used by almost every country. In many American countries as well as in The Philippines, 'Letter' is more prevalent. Paper sizes affect writing paper, stationery, cards, and some printed documents. The international standard for envelopes is the C series of ISO 269.
The international paper size standard is ISO 216. It is based on the German DIN 476 standard for paper sizes. ISO paper sizes are all based on a single aspect ratio of the square root of 2, or approximately 1:1.4142. There are different series, as well as several extensions.
The following international paper sizes are included in Cascading Style Sheets (CSS): A3, A4, A5, B4, B5.
The base A0 size of paper is defined as having an area of 1 m2 and a side ratio of 1 by √2, making the A0 paper size exactly m × m. Rounded to the nearest millimetre, that is 841 by 1,189 millimetres (33.1 in × 46.8 in).
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