We learned in lesson 1 that most stamps have a denomination (that is, an indicated value). Usually this is the form of a numeral and currency; sometimes it is a letter or word denoting a specific class of mail. For example, most current U.S. stamps bear the word “forever,” meaning that the stamp will always be usable at the then-prevailing first-class letter rate (as of 2020, 55 cents).
Stamps are available in different denominations to allow postal customers to make up different rates or to pay for different services; postal rates also change over time, which require the periodic issue of new stamps. Postal rates and services can be the basis for significant specialized collections.
Some stamps are dedicated to a specific postal service or purpose. The stamps not only pay specific rates, they help to identify mail intended for a specific service. The following list is not exhaustive.
Semi-postal: Also called charity stamps, these stamps include a charitable donation in their purchase price and, therefore, cost more to buy from the post office than the value the stamp bore for postage. Many countries indicate their semi-postals by the use of two denominations, such as “10c + 5c.”
Airmail: Many countries have issued special stamps for airmail service. The United States issued separate airmail stamps from 1918 until 1993; some more modern stamps are treated as airmails because they pay for international mail delivery carried by air. At some times in the past, regular stamps could be used to pay for airmail, but airmail stamps could not be used to pay letter rates.
Postage Due: Postage due stamps were used by a post office to show underpayment of fees on mail that was not correctly paid when sent.
Special Delivery: Special delivery stamps indicated the payment of an extra fee to secure prompt delivery of a mail item.
Official: Official stamps were used only by government offices as an accounting measure.
Registration: Many countries have issued stamps to pay for the special service of registration, which provides additional security for valuables sent in the mails.
Revenue: Stamps are also used to show the payment of a wide variety of taxes (from everything from stock transactions to taxes on bottles of wine and beer). Revenue stamps are a huge field all to themselves.
Collectors occasionally have to remember that the postal service issues stamps to meet the needs of its customers. Stamp collectors are important, since they often purchase stamps and save them rather than using them to send letters. But the designs, paper, glue, printing methods and so on, are all focused on customers who use stamps for their intended purposes.