Plate number is the number of the printing plate (or plates) printed on postage stamp. The plate number typically appears as one or more small digits in the margin at the bottom of the stamp. Although most plate numbers are composed of just numbers, both a combination of letter and number may be used. In the case when a letter is represented in the plate number, an upper case letter usually appears at the beginning or at the end of the sequence with the former serving as a good identifier of the stamp printer.

Stamp with Plate Number in the margin of the stamp
Stamp with Plate Number
Stip of 3 Stamps with Plate Number
Block of Stamps with Plate Number