Did you know that in the ISO paper size system, the height-to-width ratio of all pages is the square root of two (1.4142 : 1)? I had no idea, but it explains a lot; particularly why folding a sheet of A4 in half produces two sheets of A5, etc. Believe it or not, such a convenient format was not established arbitrarily, but has in fact a mathematical basis.

Markus Kuhn has an interesting site that tells you everything you were wondering about international standard paper sizes.