Deltiology (from Greek ?, deltion, diminutive of , deltos, "writing tablet, letter"; and -, -logia) is the study and collection of postcards. Professor Randall Rhoades of Ashland, Ohio, coined a word in 1945 that became the accepted description of the study of picture postcards. It initially took about 20 years for the name to appear in a dictionary. Compared to philately, the identification of a postcard's place and time of production can often be an impossible task because postcards, unlike stamps, are produced in a decentralised, unregulated manner. For this reason, some collectors choose to limit their acquisitions to cards by specific artists and publishers, or by time and location.
There are some general rules to dating when a postcard was printed. Postcards are generally sent within a few years of their printing so the postmark helps date a postcard. If the card is original and not a reprint, a postcard's original printing date can be deduced from such things as the fashions worn by people in the card, the era in which the cars on the street were made, and other time-sensitive clues. Postcards produced by the Curt Teich Company can be dated more exactly if the stamp box on the reverse is visible, since the company printed a date code within the stamp box.
Picture postcards (PPCs) can be assigned to "the Golden Age of Postcards" (1898-1919), the time of the linens (circa 1930-1950), or to the modern chromes (after 1940). Modern chromes are color photographs and thus differ from photochromes generated from black and white photographs before c. 1915. PPCs can also be differentiated on the basis of other features: undivided backs are typical for c. 1901-1906, and divided backs for c. 1907-1915, while white border cards are common from c. 1915-1930.
Postcards are collected by historical societies, libraries and genealogical societies because of their importance in research such as how a city looked at a particular time in history as well as social history. Many elementary schools use postcards to teach children geography. Postcard pen pal programs have been established to help children in language arts.
Deltiologists, as postcard collectors are called, collect for a variety of reasons. Some are attracted to the postcards themselves, then narrow down their interests. Others are interested in something in particular, such as ballet, then decide to collect ballet-related postcards as a way to augment their interest in ballet.
One of the most popular areas of collecting is "town views" - actual scenes from a particular town or region. Most collectors of town views start by collecting views of the town where they reside or the town where they grew up. With many small towns having hundreds of different cards available, it is an area that can keep a collector busy for years.
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The hottest cards in the town view category are real photo postcards (RPPCs), postcards printed on actual photographic paper using the photographic process rather than printed cards created on a printing press. The attraction of a RPPC is that the scene shown is an actual scene from the past whereas a printed card can often be an artist's conception of how the particular scene appeared. RPPCs are usually much rarer than printed cards as printed cards were easier to produce in large numbers. The most reliable method of differentiating the two is with a magnifying glass. A printed card viewed through a glass will display a series of dots used to create the image that are lacking in a RPPC. Another way to identify a RPPC is by information on the back of the card. Older RPPS may also display a silver sheen in the darker areas when viewed at an angle due to the silver used in the early photographic process.
Collectors may find PPCs at home in boxes, attics, or scrapbooks, generate their own on trips and vacations, and acquire them from stores, flea markets, purchasing on the Internet, or other collectors.
A number of artists have become recognized for the creation of postcards and certain publishers specialize in the production and printing of PPCs.
Worldwide, deltiology is the third largest collecting hobby after stamp collecting and coin/banknote collecting. Postcard clubs may be found in many countries; and these clubs, as well as related organizations, frequently host postcard shows. Online postcard clubs have also become popular. They mainly focus on providing their members with catalogs and features for tracking their collections and interacting with each other.