Postal history grew out of philately. As that discipline developed, philatelic students discovered that understanding and authentication of postage stamps depending on knowing why postal administrations issued particular stamps, where they were used, and how. For instance, a stamp apparently used before any other stamp of its type could be proved a forgery if it was postmarked at a location known not have received any stamps until three weeks later.
Postal history has since become a specialty in its own right. There is much that is still not known about the workings of postal systems, and millions of old covers have survived, constituting a rich field of "artifacts" for analysis.
See also: timeline of postal history