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 alongside the name of the printer.
Stamps printed in several colors will have several plate numbers in the sheet margin, one for each color. Stamps printed in large quantities may have multiple plate numbers.
The usual practice in plate block collecting varies by country and era; the classic 20th century United States plate block consists of a 2x2 block in the corner of the sheet. The ambitious collectors will seek to own blocks displaying every known plate number for the stamp; specialized stamp catalogs will list these.