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 aerograms, however. Cuba, for instance, has issued a number of colorful lettersheets that were designed for internal, non-airmail, use in Cuba. Letter sheet 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