Paper comes in different sizes. These are sizes A, B and C. Size A runs from size 4A0 to A10, size B runs from B0 to B10 while size C has different sizes all the way from size C0 to size C10. Paper measurements for length and width are stated in terms of millimetres (mm) rounded off to the nearest mm.
A simple way to fathom the different sizes is to start with a single leaf of printing paper or foolscap. That is a size 4A. Lengthwise, fold it into two equal parts to come up with a size A5 (incidentally, the dimensions of a shorthand notebook) and so on all the way to a size A10. Again, starting with a leaf of printing paper or foolscap, place another, lengthwise, besides it. You now have the dimensions of a size A3. Repeat onwards to a size 4A0, which is roughly four square metres.
B stands for books, with a size B6 roughly being the dimensions of a conventional paperback front cover. As with a size A, doubling the dimensions gives you a size B5 onwards while halving the dimensions gives you a size B7 and so forth.
C stands for container or casing, which in our case, translates to envelopes. A size C roughly corresponds with its size A counterpart but with generous allowances to comfortably fit the corresponding size A document. For example, a letter written on a foolscap, a size 4A, fits snugly in a size 4C envelope. Then again, should the letter be folded, lengthwise, equally it can then be contained in a size 5C envelope, thus reducing the cost of the envelope needed to contain it.
For more information on paper size, visit http://www.papersizes.org/.