By 1977, A4 was the standard letter format in 88 of 148 countries.Today the standard has been adopted by all countries in the world except the United States and Canada.
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. Rounded to the nearest millimetre, that is 841 by 1,189 millimetres (33.1 in × 46.8 in).
Its D format sits between a B format and the next larger A format (just like C sits between A and the next larger B).
The remaining formats fit in between all these formats, such that the sequence of formats A4, E4, C4, G4, B4, F4, D4, H4, from one size to the next.
Many paper size standards conventions have existed at different times and in different countries.
The Business Planning Guide - A4 Writing Paper
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.This means that C4 is slightly larger than A4, and slightly smaller than B4.The practical usage of this is that a letter written on A4 paper fits inside a C4 envelope, and both A4 and C4 paper fits inside a B4 envelope.This interesting arrangement of dimensions is also very useful - not only it forms a geometric progression with easy to remember formulae, it also has that each consecutive pair of values (like a sliding window of size 2) will automatically correspond to the dimensions of a standard paper format in the series.The tolerances specified in the standard are The German standard DIN 476 was published on 18 August 1922 and is the original specification of the A, B and C sizes.However, in some countries in the Americas as well as in the Philippines, the North American series of paper sizes such as 'Letter' and 'Legal' is more prevalent. 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.The first and the second editions of DIN 476 from 19 also included a D series.DIN 476 provides for formats larger than A0, denoted by a prefix factor.In particular, it lists the formats 2A0 and 4A0, which are twice and four times the size of A0 respectively.However, ISO 207 notes 2A0 and 4A0 in the table of Main series of trimmed sizes (ISO A series) as well: "The rarely used sizes [2A0 and 4A0] which follow also belong to this series." generalized the ISO system of A, B, and C formats by adding D, E, F, and G formats to it.