duodecimo

1. The size (5 by 7³/₄ inches) of book pages formed by folding single sheets from a printing press into 12 leaves each.
2. A book composed of pages of this size.

[Latin (in) duodecimō, (in) a twelfth, ablative of duodecimus, twelfth. See duodecimal.]

duodecimo [dew‐oh‐des‐i‐moh], a small size of book in which the page size results from folding a standard printer's sheet of paper into twelve leaves (i.e. 24 pages). Abbreviated as 12mo, it is thus sometimes called ‘twelvemo’. See also folio, octavo, quarto.