Illustrated envelopes - illustrated or imprinted envelopes often used for advertising and propaganda
Imitation - a stamp produced by the postal administration deliberately imitating a design when the original plates are no longer available for reprints. An imitation might differ from the originals in paper, gum, size, color, and design detail. Imitations are produced to fill gaps in official collections and exhibitions as well as for sale to collectors.
Imperforate - the lack of some separation device between stamps such as perforation holes or rouletted slits that allow for easy tearing. Imperforate stamps have to be cut apart by scissors or blades. Most early issues were imperforate, and had to be hand-cut by the postmaster.
Imprint - any detail of a stamp's design printed at the time of the original stamp's production; inscription with name, initials, etc., usually of the producers of the stamps. The latter is found in the margins of the sheets of the stamps.
Intelpost - technology that allows customers to send a facsimile transmission to foreign countries via the Postal Service
International reply coupon - receipts issued by member nations of the Universal Postage Union for return-postage payment. The certificates may be exchanged in foreign countries for local stamps.
Interrupted perforation - a system of perforation which adds strength between stamps on a coil by removing several pins from the perforation machines, thus creating wider spaces between the holes
Inverted frame - a term used to describe a misprint that leaves part of the image upside down or inverted. Occasionally, a single cliche is inverted in the plate, resulting in one color producing a tete beche in a multiple piece. That is, the inversion creates a pair of stamps connected together with one stamp right side up and the other upside down.
Inverted Jenny - a misprinted U.S. postage stamp showing an inverted image of a blue airplane. The error occurred on the 24-cent airmail stamp of 1918. Only one sheet of one hundred inverted center stamps was sold across the post office counter, and no other examples have been discovered by the public. The image attached to this record shows inverts from that single sheet, which were reunited during an exhibit at the National Postal Museum in the summer of 1996. For other information, photographs, and/or articles pertaining to this stamp, please refer to U.S. Design File C3.