LetterAgent
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A double-window envelope has two clear windows: the recipient address shows through the larger lower window and the sender address shows through the smaller upper window, so neither address is printed on the outside of the envelope itself, and both stay readable through the film.
The larger window sits in the lower center of the envelope face, where a delivery address normally goes, and the recipient's address shows through it. The smaller window sits in the upper left corner, where a return address normally goes, and the sender's address shows through that one. The windows are covered with a clear film, so the addresses stay readable while the paper inside stays protected from rain and handling.
This layout only works if the letter inside is folded and positioned so both addresses land exactly behind their windows. That is why businesses that use window envelopes print the letter with the addresses in fixed spots on the page. Fold the letter differently, or print it on the wrong paper size, and one or both addresses slide out of view. A window envelope with a shifted address is worse than a plain envelope, because the carrier sees a blank window instead of a readable address.
A single-window envelope shows only the recipient address; the sender still has to print or write a return address on the outside. A double-window envelope removes that step: the sender's address is printed on the letter itself, in the spot that lines up with the upper-left window, and it doubles as the return address. For high-volume senders, that saves a printing pass and removes a mismatch risk, since the return address can never disagree with the letter it belongs to.
The return-address behavior matters when mail cannot be delivered. If the recipient has moved or the address is wrong, the postal operator reads the sender address through the upper window and returns the piece there. The window film can make a return address slightly harder to read than ink printed directly on the envelope, but in practice the address is printed on the same page as the letter, in a normal font, and sorting equipment reads it well.
Standard business window envelopes are sized for a standard letter sheet folded in thirds. In the US, that is the usual number 10 envelope with a letter-size sheet; in Canada, the equivalent business envelope takes a letter-size sheet folded the same way. The fold matters as much as the size: a sheet folded in thirds puts the top third, which carries the sender address block, behind the upper window, and the address block lower on the page behind the main window. Fold it in half instead and the alignment breaks.
Multi-page letters work the same way, as long as the first page carries both address blocks in the right spots and the whole stack folds neatly. A stack too thick to fold crisply will shift inside the envelope, and shifting is the enemy of window alignment. Senders who mail thick letters in window envelopes often find the recipient address half-visible, which is enough to delay the piece while a person reads it by hand.
| Envelope type | Windows | Return address | Common use |
|---|---|---|---|
| Plain envelope | None | Printed or written on the outside | Personal mail, invitations |
| Single-window envelope | One, for the recipient | Printed or written on the outside | Invoices, statements |
| Double-window envelope | Two, recipient plus sender | Shows through the upper window | Business letters, bulk mailings |
They can, in two specific ways. First, the film over the window can reflect light into the address reader, though modern readers handle this well. Second, if the letter shifts in transit, the address can drift partly out of the window, forcing manual handling. Both problems are rarer than senders fear, and both are controlled by folding the letter so it fits snugly, with no room to slide.
One more quirk: window envelopes and handwritten mail do not mix. Handwriting the address on the letter so it shows through the window works in theory, but handwriting rarely lands precisely enough to sit fully inside both windows. Window envelopes are a printed-mail tool. If you are handwriting a letter, use a plain envelope and write the addresses on the outside.
LetterAgent mails every letter in a standard double-window business envelope. Your letter is printed on standard letter paper, single-sided, and folded so your sender address shows through the upper window as the return address and the recipient's address shows through the main window. You describe the letter to your AI assistant, approve the exact quoted price before anything is sent, and the printed letter is mailed to a US or Canadian address. No account or signup is needed.
Ready to mail it? Send your letter with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is printed and mailed.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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