Whether they are issued by government postal systems or private carriers, stamps are at the center of philately. They are the receipt for pre-payment of a specific level of service afforded the customer. Typically, stamps are made from special paper with a national designation and denomination on the face, and a gum adhesive on the reverse side. They reveal the mailing status and the country of origin of the cover on which they are affixed. They symbolize the authority of the state or agency handling public communication. They also serve as icons for the age. Postage stamps have been carrying the mails of the world to their destinations since the 1840's. Before this time, ink and hand-stamps, usually made from wood or cork, were often used to frank the mail and confirm the payment of postage. The first adhesive postage stamp, commonly referred to as the Penny Black, was issued in the United Kingdom in May 1840. The invention of the stamp was a part of the attempt to reform and improve the postal system in the United Kingdom.