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I was wondering that too! I got to the end and still no explanation of 210×297mm. Well, it turns out that it's based on the fact that A0 is defined as having a

I was wondering that too! I got to the end and still no explanation of 210×297mm. Well, it turns out that it's based on the fact that A0 is defined as having an area of ~1m², then each subsequent size halves the area. So A4 has dimensions that give an aspect ratio √2 and an area of ~1/16m². A0: 1,189 mm × 841 mm = 999,949mm² ≈ 1m² ... A4: 297 mm × 210 mm = 62,370mm² ≈ 1/16m² See https://en.wikipedia.org/wiki/Paper_size#A_series https://en.wikipedia.org/wiki/Paper_size#A_series : "The A0 base size is defined as having an area of 1 m² ... rounded to the nearest millimetre, 1,189 mm × 841 mm".

maweki 4y ago
That's a beautifully short definition, I think. "Base-size has 1sqm area. Halving does not change ratio of the sides."
niklasrde 4y ago
You may have skimmed over this bit in the article then: "the international A-size system now starts with A0 paper, with sides in the correct 2‾√ ratio, but with exactly 1m2 of area, or as near as you can get with whole millimetre side lengths (1189mm 841mm, to be precise). Then folding or cutting it a few times until it is conveniently desk/folder sized gets you to A4, hence the ‘4’ bit in the name."
- richdougherty 4y ago
  Yes I did! Thanks for pointing it out and very politely too. :)