In philatelic terminology a letter sheet, often written lettersheet, is a sheet of paper that can be folded, usually sealed, and mailed without the use of an envelope, or it can also be a similar item of postal stationery issued by a postal authority. 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.

Sir Rowland Hill, KCB, FRS was an English teacher, inventor and social reformer. He campaigned for a comprehensive reform of the postal system, based on the concept of Uniform Penny Post and his solution of pre-payment, facilitating the safe, speedy and cheap transfer of letters. Hill later served as a government postal official, and he is usually credited with originating the basic concepts of the modern postal service, including the invention of the postage stamp.