rarity of their subject. Recently, an increased demand for postcards has provoked an increase in prices. For example, the price of certain postcards at online auctions can reach $1000, while most postcards would not reach $5. Identification 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 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 1915. Picture postcards 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. Practice 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. Collectors may find picture postcards 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 picture postcards. Worldwide popularity Worldwide deltiology is the third-largest collecting hobby after stamp collecting and coin/banknote collecting. Postcard clubs