As a stamp collector you may need to know many different specification of a stamp to identify it correctly. For instant, many stamps depict the same picture but have different stamp specification. These stamps may have been printed in different formats, different colors, different perforation, different sizes, different printing process, and others. To help identify these stamps you need a stamp catalog. A stamp catalog will have a list of stamps issued from a certain country or countries with the specification of each stamp listed under a picture of a stamp. One very popular stamp catalog used in the USA today is Scott's catalogs. Scott prints updated stamp catalogs each year for the USA and many foreign countries. Each stamp listed in Scott's stamp catalogs is assigned a Scott reference number. The reference numbers are use by stamp dealers and collectors all over the world to communicate with each other about a certain stamp. For example, a stamp printed in the USA issued on December 15, 1979 with the name Flag will have a Scott number of 3000. The Scott number 3000 is a stamp reference to other dealers and collectors. By knowing this number its easy for a dealer or collector to look up this stamp and its specification. An example of Scott's stamp catalog listing is shown below: