ISO 216 defines the A series of paper sizes based on these simple principles:

The height divided by the width of all formats is the square root of two (1.4142).

Format A0 has an area of one square meter.

Format A1 is A0 cut into two equal pieces. In other words, the height of A1 is the width of A0 and the width of A1 is half the height of A0.

All smaller A series formats are defined in the same way. If you cut format An parallel to its shorter side into two equal pieces of paper, these will have format A(n+1).

The standardized height and width of the paper formats is a rounded number of millimeters.

A4 is 210 X 297. Isn't that lovely? 8 1/2 X 11 looks nicer, too.

The Japanese JIS P 0138-61 standard defines the same A series as ISO 216, but a slightly different B series of paper sizes, sometimes called the JIS B or JB series. JIS B0 has an area of 1.5 mÂ², such that the area of JIS B pages is the arithmetic mean of the area of the A series pages with the same and the next higher number, and not as in the ISO B series the geometric mean. For example, JB3 is 364 Ã— 515, JB4 is 257 Ã— 364, and JB5 is 182 Ã— 257 mm. Using the JIS B series should be avoided. It introduces additional magnification factors and is not an international standard.

**This all makes perfect sense to me!**