The envelope sizes may vary from tiny to huge, and there are dozens of them of sizes available. Though you'll hardly use more than 4-5 most common sizes in your everyday practice, it's a good to have an understanding of other possible styles and sizes.

The US (or Standard) envelopes are divided in several groups by their style and application. The sizes are listed in inches, the shortest dimension first. These are the most popular envelopes for business correspondence, that we send and receive every day. They are mostly used for letterheads, invoices, statements, checks, writing stationery and direct marketing mailings. Note: The 7 3/4 envelope with pointed flap is also known as Monarch. Mostly used for announcements, invitations, cards, small booklets, brochures or promotional pieces. These are more formal envelopes, are deeper and typically use a pointed flap. These envelopes are mostly used for sending catalogs, folders and the like. This style of envelope is mainly used for catalogs, magazines and reports. There are also Metal Clasp, Coins, and probably other styles, which are not listed here because they are either rare, too small or too large, or not suitable for printing (such as Metal Clasp). Window envelopes are most often used for bills, though they're also popular for marketing purposes. They are cost-efficient, since the name and address shows through the window, eliminating the need to address the envelope. Normally translucent plastic is used as the window material.

Windowed envelope address template