The standard was proposed by mathematician Lazare Carnot. In addition to A series, there is also B series (and C series). They all have the same aspect ratio √

The standard was proposed by mathematician Lazare Carnot. In addition to A series, there is also B series (and C series). They all have the same aspect ratio √2 B0 is 1 metre wide, while A0 is 1 meter long. The area of B series sheets is the geometric mean of successive A series sheets. Many books are printed in size B5. When you fold A4 it fits neatly between the pages. Letter written on A4 paper fits inside a C4 envelope, and both A4 paper and C4 envelope fit inside a B4 envelope.