A window envelope is an envelope of any size with a see-through panel through which an address, printed or written on an enclosed letter, can appear. As such, window envelopes cut out the need to print address labels or onto the envelope directly: you can create a mail merge using the sendee address on the letter itself.
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The first patent for a window envelope was received by the wonderfully named Americus F. Callaghan of Chicago in 1902. The invention took hold as large offices saved on labour costs (address only typed once) and reduced address mismatches.