vertical ridges of <#gum> on the backs of <#rotpress=rotary> <#sheetstamp=sheet stamps>, created as a part of the printing process when gum was applied to stamps using rollers that had ridges engraved on them. Ridges are known in two varieties: 1/16 and 1/32 of an inch apart. The intended purpose of ridges was to make stamps adhere better. They were used in the 1920s and 30s.