More information about different paper sizes can be found here

http://www.cl.cam.ac.uk/~mgk25/iso-paper.html

''Their claim that "Inconsequent use of SI units and '''international''' paper sizes is today the primary cause for US businessess failing to meet the expectations of the global economy" (if I understand it right; nearly every word in the claim is ambiguous) strains credibility.'' Yes; it's not as though the United States is not out of step with the rest of the planet on so many other issues (litres/gallons, yards/metres, meters/metres...) that could be expected to have bigger impacts (A primary reason why U.S. -manufactured vehicles are less popular in other countries is because so few mechanics are prepared to keep around an entire set of imperial-unit tools and widgets just for the sake of the occasional U.S. vehicle that does wander in. Can't get the parts, you see - and a 10mm bolt will not fit in a 3/8-inch hole.)

and here

http://www.twics.com/~eds/paper/papersize.html

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Why is it that the US and Canada are the only ones holding out on using A4 paper sizes?  ''Because it could cost gobs of money to change?''

''If we switched overnight (a la TheEuro ?) it wouldn't bother me. Mind you, it's not that big a deal.''

What is ''paper''? ''DeadTree''
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OTOH, what the h*ll difference does it make? I can open an A4 document in GhostScript, set the paper size to Letter, and print away. 
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Or you can use non-paper-size (no paper at all). See PaperPorting (''except in the case where you've got to comply with some outdated legal requirement to maintain all records in some non-volatile format that doesn't require intermediate machinery to read'')