Definition: A perforation gauge measure perforations on stamps. This can be very important as one stamp can have several varieties with different perforations which have different values. The best gauges are made out of plastic or metal, paper gauges do exist but can be distorted if the paper is exposed to water or heat. The measurement of the perforations is based on the number of holes along a 2 centimeter length.
The stamp is set on the gauge and is moved up and down until the number of holes match. Sometimes the horizontal and vertical sides of the stamps are not perforated the same and will need to be measured separately. Stamp catalogs will often list the perforation measurements for different varieties of a stamp.