Paper is easy to take for granted, especially in the modern world, where digital is the priority. Paper printing is even more so because there isn't much you need to do with paper and ink these days.
The exact answer to this one is lost in the foggy muck of tradition, but history gives us a few clues. Like a great many things, Americans borrowed industrial paper production techniques from the ...
The common letter-sized paper used in Europe and in most locations outside of North America. At 8.3 x 11.7" (210x297mm), A4 paper is slightly narrower and a little longer than "A" size 8.5 x 11" paper ...