from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. The size (5 by 7 3/4 inches) of book pages formed by folding single sheets from a printing press into 12 leaves each.
  • n. A book composed of pages of this size.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. A size of paper, so called because it is originally made by folding and cutting a single sheet from a printing press into 12 leaves; (5 by 7¾ inches): 6.5 to 7.5 inches high, approximately 4.5 inches wide.
  • n. A sheet or page of that size
  • n. A book having pages of that size

from the GNU version of the Collaborative International Dictionary of English

  • adj. Having twelve leaves to a sheet
  • n. A book consisting of sheets each of which is folded into twelve leaves; hence, indicating, more or less definitely, a size of a book; -- usually written 12mo or 12°.

from The Century Dictionary and Cyclopedia

  • n. A size of page usually measuring, in the United States, about 5⅛ inches in width and 7⅔ inches in length, when the leaf is uncut, and corresponding to crown octavo of British publishers.
  • n. A book composed of sheets which, when folded, form twelve leaves of this size.
  • n. In music, the interval of a twelfth.
  • Consisting of sheets folded into twelve leaves; having leaves or pages measuring about 5⅛ by 7⅔ inches. Often written 12mo or 12°.


Latin (in) duodecimō, (in) a twelfth, ablative of duodecimus, twelfth; see duodecimal.
(American Heritage® Dictionary of the English Language, Fourth Edition)
Latin ablative of duodecimus ("twelfth") (from duodecim 'twelve', from duo 'two' + decem 'ten') (Wiktionary)



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  • Often pronounced "twelvemo" and written as 12mo. In rare/antique book printing, a small-format book made from sheets folded to give 12 leaves. In general, a book less than about 7 inches high (measurements vary depending on your choice of authority).

    February 20, 2007