Many posters use B-series paper or a close approximation, such as 50 cm × 70 cm; B5 is a relatively common choice for books.
The B series is also used for envelopes and passports.
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).
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 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.
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.