Paper comes in different sizes. Paper is used every day and it has been accepted as a standard but there is some interesting mathematics behind it too.
Consider size A0: The area of a A0 size of paper is exactly 1 meter square.
Interestingly, each subsequent size is half the area of the previous size. The size A1 is half the area of A0. The A2 is half the size of A1. This also expands similarly. If you want to double sizes. Two of A5 paper sizes fit exactly into size A4.
A4 is the paper size that is commonly used and it has some interesting properties. The length to width ratio is 1:root of 2 (√2), which is about 1.414.