(Digression: A4 is a sensible size of paper used everywhere except North America. Importantly, each size in the A series is half as big as the next and they all have sides in the ratio 1:√2. This means you can cut a sheet of A3 in half to make a sheet of A4, or cut a sheet of A4 in half to make a sheet of A5, and so on. You can also, for example, make an A4 rules booklet out of folded and stapled sheets of A3, which means life is a whole lot simpler for graphic designers and printers.

The reason the dimensions of A4 don't initially look sensible in either metric of US is that the area of a sheet of A0 is exactly 1mē. The area of a sheet of A4 is exactly 1/16mē.)