Today the sizes of books are usually given in inches or centimeters. The American mass market paperback, referred to as “rack size,” is 7 inches by 4⅛ or 4¼ inches, larger than the typical 1950s 6⅜ by 4 3⁄16 inches. Trade paperbacks are generally 8 by 5¼ inches.
In the 18th and 19th centuries the sizes of books had names. Most of these names included the size of a signature. A book consists of a number of folded sheets of paper. A printed sheet of paper can be folded in half to make 4 pages; this is a folio. If it is folded in half, and then folded in half again, it will have 8 pages, and so on. The number of pages on a folded sheet is always a power of 2.
One or more folded sheets are nested together to make a signature; in a hard-bound book they are sewn together through the crease. A book's signatures can be seen by viewing the spine from above. In modern printing 32-page signatures are common.
Currently even duodecimo is often pronounced “twelve-mo.”
| Double Elephant Folio | 25"+ |
| Atlas Folio | 25" |
| Elephant Folio | 23"+ |
| Folio (F) | 13" + |
| Quarto (4to) | 11″–13″ |
| Small Quarto (Sm. 4to) | 10" |
| Octavo (8vo) | 8″–9″ |
| Small Octavo (Sm. 8vo) | 7½ to 8 inches |
| Duodecimo (12mo) | 7" or slightly more |
| Sextodecimo (16mo) | 6″–7″ |
| Vigesimoquarto (24mo) | 5″–6″ |
| Trigesimosecundo (32mo) | 4″–5″ |
| Fortyeightmo (48mo) | less than 4" |
| Sixtyfourmo (64mo) | about 3" |
| Miniature | less than 3" |
The old British designations were more precise than the American, and based upon the various paper sizes (see paper) as well as signature size. An abbreviation combines an abbreviation for a paper size with an abbreviation for a signature size. For example, “Dy 4to” is the abbreviation for “demi quarto,” and “Ry Fol” for “royal folio.”
| Octavo (8vo) | Quarto (4to) | Folio (Fol) | |
|---|---|---|---|
| Foolscap (F) | 6½ × 4½ | 8½ × 6¾ | 13½ × 10 |
| Crown (Cr) | 7 × 5 | 10 × 7½ | 15 × 10 |
| Large Crown (L Cr) | 8 × 5½ | — | — |
| Demy (Dy) | 8¾ × 5⅝ | 11½ × 8¾ | — |
| Medium (Med) | 9½ × 6 | 12 × 9½ | — |
| Royal (Ry) | 10 × 6½ | 12½ × 10 | 20 × 12½ |
| Imperial (Imp) | 11 × 7½ | 15 × 11 | 22 × 15½ |
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Last revised: 8 November 2003.