<P> A special or rare postmark can substantially add to the value of a stamp . Also, in addition to everyday postmarks there are postmarks indicating the first day of issue of a particular stamp and pictorial cancellations commemorating local events, anniversaries, and the like and slogan postmarks which advertise an event or pass information to the public . (There has been a recent change to the term "pictorial postmarks" rather than "pictorial cancellations" by the USPS .) </P> <P> There are some examples of "faked covers" produced by philatelic forgers, most usually in order to increase their value, in which the postmark has been altered in some way; for example, by changing the date . </P> <P> The postmark is often considered as an official confirmation that a cover (letter, packet, etc .) mailed item was mailed at a given location at a specific date . For example, the date of the postmark can be quite important . In the United States, the Internal Revenue Service will still consider income tax returns as filed on time though it receives them late if they are postmarked on time, and this date (with, perhaps, other proofs of mailing), may have significance as regards legal filings and proofs of service (though in this case the date may viewed as "on time" if the date of the postmark is no more than one day after the date service is supposed to have been made). Entries into sweepstakes and contests, and juried art exhibitions, may likewise have a "postmark deadline," and in at least one case it might have significance regarding the date of class withdrawal . </P> <P> A postmark should not be confused with the killer which are lines, bars, etc. used to cancel a postage stamp . Neither should a postmark be confused with overprints generally, or pre-cancels (stamps that have been cancelled before the envelope or package to which they are affixed is submitted or deposited for acceptance into the mailstream, they most commonly have taken the form of a pre-printed city name on the stamp) specifically, which generally do not indicate a date . </P>

Where is a stamp placed on an envelope