Window envelopes and their inserts should be designed
so that the entire address and a postal barcode, if applicable, appear
in the window.
The address block must maintain at least a 1/8"
clearance on all sides of the window. If a barcode is printed
either above or below the address block, the address space should
have at least 1/8" clearance on the left and right edge of
the barcode and at least 1/25" clearance above and below
the barcode. The entire window of the envelope should be no lower
than 1/2" from the bottom edge of the envelope.
All window coverings must be free of wrinkles,
streaks, fogging, or colors that may obscure the address or barcode.
We recommend the use of cellophane, polystyrene, or glassine as