Have you ever wondered why A4 is the way it is? Well Matt Parker has sorted it all out for you.
Two A4's side by side make an A3. Two A3's side by side make an A2. Two A2's together make an A1 and two A1's together make a A0. Almost like magic isn't it? Well it isn't. As explained, the A4 paper scale is based on a meter and one meter is exactly one forty millionth of the circumference of the earth.