Why size is A1?

A1 is a standard paper size defined by the ISO 216 international paper size standard. It measures 594 x 841 millimeters (23.4 x 33.1 inches) and is commonly used for large posters, architectural drawings, and other documents that require a significant amount of space. The size A1 is part of a series where each size is half the area of the next larger size, promoting a consistent scaling system.