A plate number coil (PNC) is a United States postage stamp with the number of the printing plate or plates printed on it. The plate number typically appears as one or more small digits in the margin at the bottom of the stamp. The plate number may be centered or, on some coil issues, located toward the right. 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. The plate number is on one stamp out of the number of stamps printed by a single revolution of rotary printing press used to print the stamps. The interval numbers have ranged from 7 to 52. The first coil stamp was produced in USA with plate numbers printed on periodic stamps was the 18¢ Flag of 1981. Collectors specializing in PNC collecting typically save plate number coils in the following formats: Some coil issues will require collectors to save longer than even the PNC5 mentioned earlier. For example, the United States Crops coil stamps, issued in 2006, have five designs with only one of the stamp designs bearing a plate number. While some collectors will save this issue as a PNC5 and no shorter, others may choose to save it as a PNC11 so that the stamps of the same design with and without the plate number are represented.