A postcard is a rectangular piece of thick paper or thin cardboard that can be written and mailed without an envelope. Some places offer lower postage rates for postcards compared to letters. Stamp collectors distinguish between postcards, which require a stamp, and postal cards, which have pre-printed postage. The collecting and study of postcards is called deltiology. The oldest known picture postcard was mailed in 1840 from the United States to London and caricatured postal workers sitting around an inkwell.