DIN 476 (Deutsches Institut für Normung, German Institute for Standards) mainly regained proposal of Professor of Physics Lichtenberg, who explained in a letter to his colleague Beckmann in 1786 the interest and benefits of square root of 2 ratio between length and width of a paper sheet. This maintains the same proportion if one doubles the surface of the sheet along its length, or whether it is divided by 2 in its width.
The calculation is always as follows: a/b = 2b a = √2, a proportion of 1.4142. This lets say C formats are based on a homothetic system. Width of the larger size always gives length of the smaller.
C format is world-widely used, except in North America, which uses other formats. C lower formats are most often used for envelopes. Indeed, the C sizes uses dimensions that are the geometric average of A and B sizes. Slightly larger than A, they can be used for envelopes thereof.
It is said that format A(x) may fit a C(x) envelope, which may itself fit a B(x) envelope (the largest of the three).
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