The paper sizes are German, and adopted by the ISO.
A0 is 1 m^2, by area, with sides in a ratio of 1:sqrt(2).
When you fold it in half, you get the same ratio, but now A1:
A1 is 1/2 m^2.
A2 is 1/4 m^2.
A3 is 1/8 m^2.
A4 is 1/16 m^2.
etc.
So, if you buy paper, it will be, say, 80 g/m^2. Now, one page of A4 thus is 5 g.
Dealing with French units is a pain.
(as an aside, the reason I call them 'French units' is because the name 'metric' privileges them: our standard units are no less a system of measures than are they; this too is my problem with 'SI,' since our system is (or was) likewise international)
No farmer or brewer deals in units that are easier to half in American rather than metric units. They use tonnes, hundreds or thousands of litres, and large areas. Outside America, they don't need to convert between units of the same type: no acre-feet, bushels, pounds, tons, or all that crap.
Calling them "French units" sounds like xenophobia. The American system was never international, and the British system didn't extend that much further than the French, at the time of the empire.
In America, anyone who would deal solely in tonnes can deal solely in tons; anyone who would deal solely in hectolitres can deal solely in barrels (traditionally, 128 quarts); anyone who would deal solely in kilolitres can deal solely in tuns (traditionally, 1,024 quarts). No-one in America has to say '7 tuns, 2 hogsheads, 3 gallons, six quarts and a fluid oz' any more than he'd say '7.2850546 kilolitres.'
> Calling them "French units" sounds like xenophobia.
Oh, I'm not xenophobic! I just don't think they deserve a privileged position.
> The American system was never international, and the British system didn't extend that much further than the French, at the time of the empire.
The American system was used in Liberia; prior to the Russian Revolution the majority of the peoples of the world used systems of measurement substantially similar to the Anglo-American system, and could (should have, IMHO) rationalised and standardised that, rather than adopting the objectively inferior decimal principal.
Then your argument that these quantities are easily divided is irrelevant.
> 7.2850546 kilolitres
My house's water meter measures up to 99,999.999m³, or in litres up to 99,999,999 litres.
Europe other than Britain and Ireland didn't use the British system, and neither did their colonies, China, Japan, etc. All the British colonies except the USA chose the metric system after independence.
The SI system is the only one that's truly international, set up by international treaty between many countries.
I guess, but for what it's worth, I've never heard that before. It's a poor way to communicate, if communication is your goal.