Windowed envelope

Americus F. Callahan of Chicago, Illinois, in the United States, received the first patent for a windowed envelope on 10 June 1902.Originally called the "outlook envelop", the patent initially anticipated using thin rice paper as the transparent material forming the window.That material was soon replaced by glassine and by the end of the century, clear plastics; some uses omit the film entirely, leaving an open cut-out.Callahan also recommends the use of black paper, which would likewise provide an opaque background whilst simultaneously increasing the contrast with the white address blocks.Owing to the benefits in both time, cost, and quality, the windowed envelope design has become nearly ubiquitous among modern commercial mailings.
US Patent 701,839 for a windowed envelope
Modern envelope with a single window for the recipient address
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