A perforation gauge is a tool used by philatelists to determine the number of perforations the edge of a stamp has in a 2cm length. The numbers for horizontal and vertical perforations can be used to determine when a stamp was issued, even if several identical stamps were produced at other times. Many stamps made during the early twentieth century can only be identified through use of a perforation gauge and equipment for seeing watermarks. A difference of half of a perforation per 2cm on a stamp can be the difference of a stamp being worth fifteen cents and it being worth hundreds of dollars.

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