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Perforations are the rows of holes between stamps on a sheet, booklet or roll. Originally the stamps of the United States and other countries were issued imperforate - without holes. Imperforate stamps were separated by postal clerks using scissors - a tedious and time-consuming process which often mangled stamps. In the 1840s and 50s several devices were patented to perforate stamp sheets and these methods (as well as the concept of perforation) quickly spread through the stamp producing world. Alternately, some countries experimented with various forms of "rouletting" in which small cuts are made in the paper but no holes are punched. Rouletting did not produce satisfactory results and its use did not survive into modern times.
Perforation measurement provides a way to distinguish otherwise similar appearing stamps, and all major stamp catalogs will provide this information. The standard way to describe perforations is by counting the number of holes in a two-centimeter span. Typically this number falls between 10 and 12, but historically much larger and smaller numbers exist. Many times the horizontal perforation measurement is different from the vertical. In this situation, the standard is to list the horizontal measurement first (i.e. 11x10.5). Collectors measure perforations using a tool called a perforation gauge. There are several variations which all involve holding the stamp against a sliding scale and finding the place where the holes of the perforation line up with scale. There are also clear plastic gauges available which are helpful for viewing stamps still attached to envelopes ("on cover") or mounted in albums.
Early in the 20th century, there were extensive experiments with both machines that would dispense stamps and machines that would affix stamps to envelopes. In the United States, the Bureau of Engraving and Printing made imperforate sheets of stamps available to private manufactures, who would in turn perforate the stamps themselves for use in their equipment. The result is a rich assortment of "private perforations" that were produced between 1902 and 1927. These stamps are generally described by the name of the manufacturer - i.e. Schermack, Brinkerhoff etc.