In philatelic terminology a letter sheet, often written lettersheet, 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, 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.

The term lettersheet has been used to describe the unstamped folded sheet letters used before envelopes became popular. Recent academic research and conservation initiatives have termed such folded and sealed letters as "letterpackets”; however, only a relatively small number of early examples, such as the Brienne Collection (1689–1706) at The Hague, are known to exist. Envelopes were not used much before the second half of the 19th century, because most countries' postal rates calculated for the extra sheet of paper that made up the envelope, thereby increasing the cost of mailing when an envelope was used.

Pre-paid lettersheets issued by postal operators are postal stationery because they bear imprinted stamps, or indicia that indicate pre-payment, as opposed to adhesive stamps that are only printed by postal authorities. Lettersheets that require stamps to be applied have also been produced by private firms that usually have no authority for a pre-paid indicia, so postage must be paid by normal means at normal postage rates. Most countries' postal authorities have issued true lettersheets at some stage; however, most have discontinued their use, except in the form of an aerogram, due to the popularity of envelopes.

The first postal stationery item issued by a government is thought to be the AQ lettersheet issued in 1608 and showing the coat of arms of Venice. In 1790 Luxembourg produced a 25-centime lettersheet. British newspaper publishers printed colourful stamps on paper supplied by the government between 1712–1870 and Australia produced lettersheets two years before the Mulready lettersheets were issued in 1840. During this period envelopes were rarely used. New South Wales issued prepaid lettersheets in 1838 with uninked embossed stamps (making them difficult to see) to prepay postage within the town of Sydney.

Prepaid lettersheets were introduced in the United Kingdom of Great Britain and Ireland at the same time as the first postage stamps were available for use on May 6, 1840. Part of Rowland Hill's postal reforms were the introduction of prepaid lettersheets and envelopes designed by the artist William Mulready, whose name is always associated with these first lettersheets and envelopes. In the same way that the first postage stamps were issued in two values (Penny Black and Two Penny Blue) both the lettersheets and envelopes were issued in one penny and two penny values in the same black and blue colours as the same value postage stamps.

The design incorporated Britannia at the centre top with a shield and a reclining lion surrounded on either side by a representation of the continents of Asia and North America with people reading their mail in the two lower corners. Rowland Hill expected the lettersheets to be more popular than the postage stamps but the postage stamp prevailed. Many caricatures were produced by stationery manufacturers whose livelihood was threatened by the new lettersheet. Only six days after their introduction, on May 12, Hill wrote in his journal:

I fear we shall have to substitute some other stamp for that design by Mulready ... the public have shown their disregard and even distaste for beauty.

Within two months a decision had been made to replace the Mulready designed stationery and essentially they were a folly.