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> 80 gsm (grams per square metre) paper at A0 size weighs precisely 80g. A4 paper at that density therefore weighs 5g since it’s been halved 4 times

So when they mark the gsm number onto a ream of A4 sheets, is 80 the gsm of A0 or A4?

Excellent article by the way. Thank you.

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> So when they mark the gsm number onto a ream of A4 sheets, is 80 the gsm of A0 or A4?

It's the same thing, since it's the grams per square meter. A0 is 1sqm by definition, and if you tile A4s until you have 1sqm of it, you get A0 (at least if you double through the small side every time: A4 -> A3 -> A2 -> A1 -> A0)

Both. A 80 gsm ( = grams per square meter)A4 sheet has the same density as a 80 gsm A0 sheet (80 gsm= 80 gsm) but only 1/16th its weight (5g = 1/16×80g) since it only has 1/16th its area.

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