ISO paper sizes are all based on a single aspect ratio of the square root of two, or approximately 1:1.4142. The advantages of basing a paper size upon this ratio were already noted in 1786 by the German scientist Georg Lichtenberg (in a letter to Johann Beckmann): if a sheet with aspect ratio âˆš2 is divided into two equal halves parallel to its shortest side, then the halves will again have aspect ratio âˆš2. In the beginning of the twentieth century, Dr Walter Porstmann turned Lichtenberg’s idea into a proper system of different paper sizes. Porstmann’s system was introduced as a DIN standard (DIN 476) in Germany in 1922, replacing a vast variety of other paper formats. Even today the paper sizes are called « DIN A4 » in everyday use in Germany.
– Paper size, Wikipedia.