Plate block

A plate block, or plate-number block is a block of postage stamps that includes the part of the sheet margin with the serial number of the printing plate(s) used to print the stamps.

The numbering of printing plates has long been a part of quality control in the printing process. In some cases, for instance the Penny Reds of Great Britain and modern United States plate number coils, the plate numbers appear in the stamps themselves, but the more common practice is to include the number in the margin of each sheet, frequently along with the name of the printer.