How to check perforations on stamps

In some countries such as U.S.A. although you may have two stamps that appear to be the same, the perforation differences between them can make a big difference in the price. The reason usually boils down to scarcity.
Sometimes only a few stamps are created with a specific perforation variety. When collectors learn of the variety and want to add it to their collections, demand may exceed the available supply and cause prices to climb quickly. Fortunately, collectors have a fairly simple method to distinguish different perforation measurements and identify varieties.
Most stamp catalogs included information about perforation measurement for every perforated stamp. To measure perforations, collectors use a tool called a perforation gauge.
The measurement of a stamp's perforations is described and listed as the number of perforation teeth or holes that are found within the space of two centimeters. The description "perf 11" means that the perforations on each side of the stamp measure in such a way that the stamp would have 11 perforation teeth or holes within the space of two centimeters. If described as "Perf. 11.2 X 11.4." the stamp has compound perforations. The horizontal perforations are always listed first and the vertical perforations are listed second.
To measure the stamp, the collector places it upon the measuring scale in the center of the Perforation-Gauge, and slides the stamp up or down until the slanted vertical lines precisely intersect with each perforation hole or tooth on the stamp.
Many worldwide stamps have similar perforation differences between otherwise identical stamps, making perforation measurement an important part of your enjoyment and understanding of the stamp hobby