-mo

Book sizes.

The final syllable of the masculine ablative singular of Latin ordinal numbers.

Book sizes are determined by the number of leaves into which a sheet of paper has been folded. Older examples are formed on Latin ordinal numbers, such as duodecimo (Latin in duodecimo, in a twelfth, from duodecimus, twelfth), a size of book in which each leaf is one twelfth of the size of the printing sheet. More recently, equivalent terms have been formed using English cardinal numbers: twelvemo, sixteenmo, thirty-two-mo.