Envelopes come in many sizes. To find a size, click here:
http://www.leaderpaper.com/info/size_chart.php
The most common size is the #10 business envelope. Most are printed with a return address or other contact information in the top left corner, and may have a window. It is possible to print on the flap. A #9 envelope is designed to fit into a #10, and often has the return address centred and may have a postage paid “indicia” to allow return mail. One or two colours without bleeds are recommended for envelopes, as full colour is costly in quantities below ten thousand.
