Large companies buy ''precanceled'' stamps from the post office to conduct mass mailings. These stamps may not be used for any purpose other than mass mailing, and the general public can not buy them or use them again on other mailers. Although individual citizens never buy precanceled stamps, or precancels, as they are sometimes called, they come into contact with these issues often through so-called ''junk'' mail.

Precanceled stamps have long fascinated philatelists, some of whom, in 1922, formed the Precanceled Stamp Society. This Society puts out catalogs and bulletins about precancel ''must-haves.'' Dozens of countries utilize some form of the precancel. In the United States alone, there are almost 10,000 different kinds of precanceled stamps, otherwise known as ''bureau precancels.''

To save resources and printing costs, governments often overprint whole series with precanceled stamps. French postal workers, for example, stamp the word ''Affranches'' on such stamp series. Local towns get in on the precancel action, as well, and overprint series with home-grown designs. In fact, just from examining a country's precanceled stamp features, one can piece together much of that nation's postal history.

Precanceled stamps have recently fallen out of vogue with the United States Postal Service. Thanks to digital techniques and the diversification of postal requirements, the USPS has been able to subdivide its classes of mass mailers. These stamps include ''whole rate'' stamps and ''pre-sorted'' stamps. That being said, you can still come across USPS precancels, particularly in rural areas of the country.