In philatelic terminology a letter sheet, often written lettersheet, is an item of postal stationery issued by a postal authority. It is a sheet of paper that can be folded, usually sealed (most often with sealing wax in the 18th and 19th centuries), and mailed without the use of an envelope. Letter sheets derive from the form in which written correspondence was made up before the mid-19th century — letters were written on one or more sheets of paper that were folded and sealed in such a way that the address could be written on the outside.