A book whose sheets are folded into twelve leaves each. This word, which differs from both the Italian and French, is from the Latin duodecim (twelve). It is now called twelvemo, from the contraction 12mo. The term is still applied to books that are the same size as the old duodecimo, irrespective of the number of leaves into which the sheet is folded.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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