Perforation Gauge (pronounced “gage”)

Stamps can appear identical but have different perforation sizes. Measuring “perfs” with this gauge is one way collectors distinguish between similar varieties of stamps.

Perforations—the holes and teeth between stamps—made it easier to separate individual stamps from a roll or a sheet.

A sheet of four 16-cent stamp of US Air Mail insignia-a circular center with spread wings divided by perforated holes.
A 24c De Havilland biplane stamp measured against a perforation gauge. The stamp is a size 11.