Paper Size Series A
If a rectangle with aspect ratio is folded (or cut) into two equal parts, then the aspect ratio of the new rectangles remains the same. This property is used in determining paper size series A according to the international standard ISO 216. The basic format is defined as a sheet of paper measuring 1 in area. The successive sizes in the series are designated A1, A2, A3, and so on, which are produced by halving each successive rectangle parallel to its shorter side. One of the many advantages of this system is that blueprints having different sizes of the series can be conveniently folded to the same size for storing.
The longest sides for A0, A1, A2, … are the geometric progression , , , …, rounded to the nearest mm.
For further information on paper sizes see the Wikipedia entry.