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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.) 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.
There are many clubs devoted to the hobby of collecting postmarks.
The postmarks bore the initial of the particular post office or handling house it was sent from along with a separate time stamp.