In most countries in the world for office use rectangular sheets of writing paper with a side length of 210 and 297 mm, respectively. This paper is marked A4. This information is widely known. However, a curious person immediately raises a number of questions. Why 297210 mm? And why it follows the standard format A4, but not A3, or A5. Below we will clarify these points and at the same time, briefly describe why at the time, was adopted such standard formats of paper sheets. A website https://kvn201.com.ua/table-of-paper-sizes.htm you can see with the sizes of different paper sizes.At the turn of XIX-XX centuries, each firm engaged in the production of paper sheets, releasing paper sheets own special format since uniform standards for the size of paper sheets for record keeping did not exist. But the need for such a standard every year it has become increasingly clear.The problem was that not immediately able to find the optimal shape (the ratio of the lengths of the sides) for a conventional rectangular sheet of paper. Attempts to use came from the Renaissance rule of the Golden section where the side of the sheet was maintained in the ratio of 1/1,618, did not lead to a positive result, because when such a sheet is folded twice, resulting in the result page was very impractical and unattractive in appearance. Therefore, leaves these proportions were awkward to use in record keeping and/or printing.