The Scott Catalogue of postage stamps, published by Scott Publishing Co, is updated annually with hundreds of thousands of changes and lists all the stamps of the entire world . From its humble beginning as a 24-page bound pamphlet, the multi-volume set now list more than 700,000 stamps from 600 different postal entities. Because of the size of each Volume, the 2021 edition has each volume split into a part A and B. So when purchasing you are obtaining the volume set of part A and B. Scott Publishing publishes a total of eight large volumes that include six volumes containing all the countries of the world, the United States Specialized Catalog, and the 1840-1940 Classic Specialized Catalogue (covering the world for the first 100 years that stamps were issued). The numbering system used by Scott to identify stamps is dominant among stamp collectors in the United States, Mexico, Canada and through out the world. It is a must for any researcher or stamp collector