8.5×11 has been around for more than a century. 8.27×11.69 was made an international standard in the 1970s. Most of Europe had already adopted it since the 20s.
In North America, continued to use the 8.5×11 standard, because the 70s paper manufacturing in NA was already a huge industry and the prevalence of the electric typewriter and it would be unreasonable to force that industry into changing the standard size paper for an arbitrary reason.
The significant advantage of this system is its scaling: if a sheet with an aspect ratio of √2 is divided into two equal halves parallel to its shortest sides, then the halves will again have an aspect ratio of √2. A0 is 1M2.
And crucially, paper “weight” is given in grams per square meter (g/m2 or gsm), so is the same for all paper sizes.
A4 is 1/24 m2, so a sheet of 80gsm paper in A4 size weighs exactly 80/24 or 5g. The same paper in A3 weighs exactly 10g, in A5 2.5g, etc.
The USA has no system of paperweights that apply across different paper sizes. 22lb newsprint stock (36gsm) is less than half the weight of 20lb bond stock (75gsm).
There is some confusion about why this is important. If I want a specific “weight”, “thickness”, or rather a density, of paper, I can specify “60gsm” or “110gsm”, and it’s the same kind of paper whether it’s the size of a business card or the size of a bedsheet.
In everyday life, we normally use 70-80gsm to fill the printer and 350-400gsm for business cards, but you can get anything in between, and cut it into any size you want, and you know exactly what “weight”/thickness of paper you are getting.
The same is not true in the US. Because the US measures in pounds per a specific uncut size of the paper, ~60gsm in bond stock is “16lb”, in newsprint stock is “35lb”, in book stock is “40lb”. Then there’s index stock, cover stock, etc. You need tables to compare paper that comes from different stocks).Reddit user: Intergalacticspy