The idea behind the A series of measurement for standard paper sizes is so that each sheet is mathematically exactly half the size of it's preceding number. Therefore, a whole A sheet of paper has the dimensions of 841 mm by 1189 mm and the next in line, A1, is exactly half the size at 594 mm by 841 mm and so on until the smallest sheet in the A series which is A8 (although A9 all the way up to A10 is also recognized). The most frequently used size from this series in the A4 paper which became the standard document format used by the United Nations.
Very rare countries, like the US, have not yet officially adopted the A4 paper size for standard documents however. Instead, they use the standard US Letter size with the dimensions of 216 mm by 279 mm. More often referred to as 8.5 inches by 11 inches. Another frequently used size in countries which use the imperial system of measurement is the Legal sized pad which is longer than a letter sized sheet with the dimensions of 216 mm by 356 mm – or 8.5 inches by 14 inches.
The difference in these international document sizes is really only just a few millimeters, with A4 paper being slightly taller and narrower than a US Letter sized sheet. But this can lead to a few inconvenient problems however, especially when it comes to printing and photocopying documents which may be emailed back and forth across different countries. Even though back in 1974, Australia was successfully able to switch completely to the A standard used by most of the world, it is unlikely that the United States, or other countries, will do the same in the near future. Small steps are being taken to globalize the standard document size to A4 paper, if not for any other reason than convenience. Universities in the United States and Canada, for example, are beginning to make the change simply because it is easier for students to photocopy journals printed on the standard A4 size.
The variation in length and width may be subtle but it is important to know the difference between the two standard sizes of documents. The US Letter size is slightly shorter and wider than an A4 paper so layout artists preparing documents or artwork for printing should take this into consideration when preparing final artwork or documents for print.