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),
Book size once to form two leaves or four pages, are referred to as "folios" (from Latin, "foliō", ablative of "folium", leaf). Those made by printing four text pages on each side of a sheet of paper and folding the paper twice to form a gathering containing four leaves or eight pages are called "quartos" (fourths). Similarly, books made by printing eight pages of text on each side of a sheet, which was then folded three times to form gatherings of eight leaves or sixteen pages each, are called "octavos". The size of the resulting pages in these cases depends, of course,