100 - #10 - 4.125 x 9.5

An envelope is a common packaging item, usually made of thin flat material. It is designed to contain a flat object, such as a letter or card.

North American sizes:

Envelope No. 9: (3.875"x8.875")
Envelope No. 10: (4.125"x9.5")
Envelope No. 11: (4.125"x10.375")

Window envelopes have a hole cut in the front side that allows the paper within to be seen. They are generally arranged so that the sending address printed on the letter is visible, saving the sender from having to duplicate the address on the envelope itself. The window is normally covered with a transparent or translucent film to protect the letter inside. A return envelope is a pre-addressed, smaller envelope included as the contents of a larger envelope and can be used for courtesy reply mail, metered reply mail, or freepost (business reply mail). Some envelopes are designed to be reused as the return envelope, saving the expense of including a return envelope in the contents of the original envelope. The direct mail industry makes extensive use of return envelopes as a response mechanism.