While there are numerous types of perforated stamps, with anywhere from seven to 16 1/2 perforations in the space of two centimeters, there are really only three types of imperforate U.S. stamps. The first are those issued before 1857, when the first three-cent stamps bearing the profile of George Washington were printed by Toppan & Carpenter of Philadelphia and perforated using a rotary perforating machine manufactured by Bemrose & Co. of Derby, England. The second type of imperforates are errors, or stamps that were intended to be perforated but somehow managed to slip through the machinery unscathed.

A third class of imperforate stamps encompasses those issued between 1907 and 1927 on imperforate coils, which were designed for, and sold to, private vending companies such as Brinkerhoff, Farwell, Schermack (which became Mailometer), and U.S. Automatic Vending. The machines made by these companies perforated the stamps for customers as they were dispensed. In some cases, completely imperforate stamps can be found from these rolls, but many have circular or “hyphen-hole” perforations between each stamp, with no perforations on the outside edges. U.S. Automatic Vending was known for its notched stamps, in which the corners of stamps were cut off to make them easier to tear.