The dimensions of the B series paper sizes are defined by the ISO 216 international paper size standard. The B series is not as common as the A series. It was created to provide paper sizes that were not included in the A series. The B series uses an aspect ratio of 1:√2. The area of the B series paper sheets is the geometric mean of the A series sheets. For example, B1 is between A0 and A1 in size. While the B series is less common in office use, it is more regularly used in other special situations, such as posters, books, envelopes and passports.