At the beginning of the twentieth century, there were no uniform sizes for business papers and design documentation in Europe at all. Each paper mill produced its own sheet size.
The proportions of the rectangular sheet were also chosen differently. The "golden ratio" 1: 1.618, which the architects and painters of the Renaissance loved so much, turned out to be completely unsuitable for printing and publishing. When folding such a sheet in half, the proportions of the resulting page changed and became inconvenient for work.
Other proportions turned out to be more suitable for practical needs: a leaf, the sides of which belonged as a unit to the square root of two, when folded in half, gave a rectangle with the same ratio. In other words, it was similar to the original one.
Dr. Walter Porstmann, a German engineer, mathematician and theorist of standardization in industry, one of the founders of the German DIN system, proposed to standardize paper sizes based on a sheet with an aspect ratio of 1: 1.4143 and an area of one square meter.
The draft of the German Manufacturing Standards Committee was published on August 18, 1922. The original format was designated A0. The results of folding it in half are A1, A2, and so on, respectively. Thus, A4 size is 1/16 of a large A0 sheet.