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A postcard or post card is a rectangular piece of thick paper or thin intended for writing and mailing without an envelope. Shapes other than rectangular may also be used. There are novelty exceptions, such as wood postcards, made of thin wood, and copper postcards sold in the Copper Country of the U.S. state of Michigan, and coconut "postcards" from tropical islands.

In some places, one can send a postcard for a lower fee than for a letter. Stamp collectors distinguish between postcards (which require a stamp) and postal cards (which have the postage pre-printed on them). While a postcard is usually printed by a private company, individual or organization, a postal card is issued by the relevant postal authority.

The world's oldest postcard was sent in 1840 to the writer Theodore Hook from Fulham in London, England. The study and collecting of postcards is termed deltiology.

Cards with messages had been sporadically created and posted by individuals since the beginning of postal services. The earliest known picture postcard was a hand-painted design on card, posted in Fulham in London to the writer Theodore Hook in 1840 bearing a penny black stamp. He probably created and posted the card to himself as a practical joke on the postal service, since the image is a caricature of workers in the post office. In 2002 the postcard sold for a record £31,750.

In the United States, the custom of sending through the mail, at letter rate, a picture or blank card stock that held a message, began with a card postmarked in December 1848 containing printed advertising. The first commercially produced card was created in 1861 by John P. Charlton of Philadelphia, who patented a postal card, and sold the rights to Hymen Lipman, whose postcards, complete with a decorated border, were labeled "Lipman's postal card". These cards had no images.

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