Paper size a number of different sizes were defined for large sheets of paper, and paper sizes were defined by the sheet name and the number of times it had been folded. Thus a full sheet of "royal" paper was 25 × 20 inches, and "royal octavo" was this size folded three times, so as to make eight sheets, and was thus 10 × inches. Imperial sizes were used in the United Kingdom and its territories. These sizes are no longer commonly used since the UK switched to ISO sizes. Many of these sizes were only used for making books (see bookbinding),
Letter sheet on the outside. 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 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