Here is an image from Wikipedia that shows the difference between A4 and US Letter and Legal sizes. We always have to do things different here.
If you look at your printer or scanner, you will notice that there is a little extra room in the paper feed tray and scanner bed size. This is because most of these devices are made to the slightly wider/shorter A4 specification.
http://upload.wikimedia.org/wikipedi...lustration.png
I never realized until I saw this picture that the various A-sizes came from the successive cutting in half of a larger standard size of paper... the A0.