Paper Sizes

Architects have slightly different ratios for width and height than either ANSI (e.g., 8 ½ x 11) or ISO. Size ‘A’ is 9 x 12 inches.

ISO papers sizes include the common A series with a ratio  of square root of 2. If folded in half, the ratio remains the same.

Additionally, ISO standards include a less common B series, used for a variety of special situations, and a C series, which is used for envelopes.

 

Source: http://en.wikipedia.org/wiki/Paper_size