In the realm of commercial and official envelopes, let’s start with #9 and #10 envelopes. The #10 envelope is the one you receive most often in the mail. It holds standard tri-folded letter paper (8.5” x 11”).
If you receive a marketing package, and the sender wants you to fill out a form (or remit payment) and send it back, usually this goes in a #9 envelope because this will fit comfortably (with other direct mail items) in the #10 envelope, which is also called an “outgoing envelope.”
Both the #10 and #9 envelopes are usually made of 24# stock (usually white wove, comparable to 60# text). Obviously your printer has latitude in paper stock, but if you can print on pre-made envelopes, they will be cheaper, and you know they will be acceptable to the US Post Office.
The #10 envelope comes in two “flavors,” regular and window envelopes. If you will inkjet the recipient’s address on the envelope or on a label, the regular envelope will be your proper choice. However, if your mailing insert (usually a letter) has the recipient’s address on the front, your envelope printing supplier can fold the letter in thirds and insert it into a window envelope with the address visible through the window in the envelope. This makes labeling the #10 envelope unnecessary. These windows come in a variety of sizes and positions on the envelopes.
Regular and window envelopes come in many, many other sizes (noted in envelope charts as 6¾, 7, 7¾, Monarch, and the like). On the charts, each has a number and a size (7¾, for instance, is 3 7/8” x 7 1/2”).
It’s very important to choose an envelope that is large enough for your insert. Many envelope charts also include notations of the insert size as well as the envelope size. You want to have a 1/8” clearance on the top and on either side of the insert when the insert is in the envelope. An A-1 envelope, for instance, is 3 5/8” x 5 1/8”. It will accept 3 1/2” x 4 7/8” inserts. (That is, when the insert is in the envelope, there’s 1/8” of leeway on the left and right plus 1/8” leeway at the top opening, or “throat,” of the envelope.)
Read more at https://www.printindustry.com/blog/2020/04/commercial-printing-tips-for-printing-envelopes/