A4 is an ISO size. Used everywhere except the USA. It's exactly a ratio of Square Root of 2. Thus A3 is two A4 side by side. An A4 can be cut in half and gives A5.
https://en.wikipedia.org/wiki/ISO_216
Thus it's not just German DIN. The USA Letter Size is a bit wider and shorter. No doubt that's why laptop screen makers thought it acceptable to migrate to Video sized 1920 x 1080 rather than 1920 x 1200. A4 PDFs don't fit well on a 1080 line screen, but US Letter Size PDFs are OK.
Also the so called HD 1920 x 1080 is based on Analog 1125 lines, the Japanese HD version of NTSC (525 lines) which has 480 visible. Hence original VGA was 640 x 480. The digital equivalent of PAL is 768 x 576, so the 720p version of TV HD is a big improvement for the USA & Japan, but marginal for the rest of the world.
Despite the majority of people in the World using A4, every laptop and laser printer has the US letter size as the default.
It's not even a German idea or even recent!
Quote:
The oldest known mention of the advantages of basing a paper size on an aspect ratio of √2 is found in a letter written on 25 October 1786 by the German scientist Georg Christoph Lichtenberg to Johann Beckmann.
The formats that became ISO paper sizes A2, A3, B3, B4, and B5 were developed in France. They were listed in a 1798 law on taxation of publications that was based in part on page sizes.
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