There are a series of common terms to describe the sizes of modern books. These range from folio, which is the largest, to quarto, and octavo, which are respectively smaller. Book sizes are usually measured by the height and width of a leaf, or the height and width of its cover. The size and proportions of a book depends on the size of the original sheet of paper that was used to produce the book. The most common book sizes today are octavo and quarto. However, many books are still produced in larger and smaller sizes. There are some other terms for book sizes. These include elephant folio (up to 23 inches tall), atlas folio (25 inches tall), and a double elephant folio (50 inches tall). The B-format, such as the Penguin Classics, are 129mm by 198mm, which are a common paperback size in the UK.