Lettersheet

In philatelic terminology a Letter sheet, often written lettersheet, is nowadays 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 century, and mailed without the use of an envelope. Letter sheets derive from the form in which written correspondence was made up before the mid-1800s when 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. They were literally a letter on a sheet (of paper).
The term lettersheet has been used to describe the unstamped folded sheet letters used before envelopes became popular. Envelopes were not used much before the late 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; an extra postal charge was made for the extra sheet of paper.
Pre-paid lettersheets issued by postal operators are postal stationery because they bear imprinted stamps, or indicia, indicating 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 and usually have no authority for a pre-paid indicia, so postage must be paid by normal means at normal postage rates. Most country's postal authorities have issued true lettersheets at some stage, however, most have discontinued their use, except in the form of an aerogram and due to the popularity of envelopes.
online source:
1d Mulready Lettersheet
2d Mulready Lettersheet
Mulready Stationery Envelopes, Lettersheets, Parodies, Lampoons, Caricatures and Exhibit Pages
National Postal Museum, Washington, D.C. - Stamps That Changed the World
Mulready example page Vol. II - British Postal Museum & Archive
Mulready example page Vol. IX - British Postal Museum & Archive