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For a standard #10 envelope, tri-fold the letter: fold the bottom third up, then the top third down, so it opens like a Z and slides in cleanly. For a window envelope, fold so the address panel lands in the window. Crease each fold firmly, and fold the printed side inward so the text stays private.
The order matters more than people think. Bottom-up first, then top-down, leaves the top panel (with your letterhead or opening paragraph) on the outside of the fold. Fold it the other way and the letter opens upside down, which is a small thing that makes a letter look careless.
Window envelopes show the recipient's address through a cutout, so the fold has to place the address panel exactly behind the window. Type the address in the standard spot (about a third of the way down the page), tri-fold bottom-up then top-down as above, and check the alignment before sealing. If the address sits even half an inch off, the window shows blank paper and the envelope is undeliverable. When in doubt, hold the folded letter up to the envelope and look through the window before you seal it.
Legal paper (8.5x14 inches) is too long to tri-fold into a #10, so it gets the legal fold: one fold straight across the middle. The folded sheet is 8.5x7, which slides into a #10 with room to spare.
| Fold | Paper | Envelope | When to use it |
|---|---|---|---|
| Tri-fold (letter fold) | 8.5x11 | #10 | The default for business letters |
| Legal fold (half fold) | 8.5x14 | #10 | Contracts and legal documents on legal paper |
| Quarter fold | 8.5x11 | A2 | Short notes and personal letters in small envelopes |
| No fold | 8.5x11 | 9x12 flat mailer | Resumes, certificates, anything that should stay crisp |
Three of them. First, folding the printed side outward, which exposes your text to anyone who glimpses the envelope's contents and looks sloppy. Second, creasing with a fingernail on heavy paper, which gives a wavy fold; use a ruler edge. Third, overstuffing: more than about five folded sheets in a #10 makes a fat envelope that will not seal cleanly and may pop open in the mail. Past that point, use a larger envelope or a flat mailer.
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