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.

Paper sizes affect writing paper, stationery, cards, and some printed documents.

The area of B series sheets is the geometric mean of successive A series sheets. So, B1 is between A0 and A1 in size. As a result, B0 is 1 metre wide, and other sizes in the B series are a half, a quarter or further fractions of a metre wide.

While less common in-office use, it is used for a variety of special situations. 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. The B-series is widely used in the printing industry to describe both paper sizes and printing press sizes, including digital presses. B3 paper is used to print two US letter or A4 pages side by side using imposition; four pages would be printed on B2, eight on B1, etc.

More Info: en.wikipedia.org