Standard envelope sizes can vary by country, but the standard business envelope size in North America is 4 1/8 by 9 1/2 inches, or the #10 envelope. Other common envelope sizes include catalog envelopes measuring 6 by 9 inches, 9 by 12 inches and 10 by 13 inches.
Envelopes come in many styles and formats to complement a wide variety of paper types, finishes and weights. One common envelope style is the window envelope, which has a rectangular, plastic screen to show the recipient's name and address printed on the letter inside. Some envelopes include protective padding and security features, all made in various sizes, and some are designed for airmail or other shipping methods.