[ ok-tuh-des-uh-moh ]
/ ˌɒk təˈdɛs əˌmoʊ /
Save This Word!

noun, plural oc·to·dec·i·mos for 2.

a book size of about 4 × 6 1/4 inches (10 × 16 centimeters), determined by printing on sheets folded to form 18 leaves or 36 pages. Symbols: 18mo, 18°
a book of this size.


in octodecimo.



Were you ready for a quiz on this topic? Well, here it is! See how well you can differentiate between the uses of "was" vs. "were" in this quiz.
Question 1 of 7
“Was” is used for the indicative past tense of “to be,” and “were” is only used for the subjunctive past tense.

Meet Grammar Coach

Write or paste your essay, email, or story into Grammar Coach and get grammar helpImprove Your Writing

Meet Grammar Coach

Improve Your Writing
Write or paste your essay, email, or story into Grammar Coach and get grammar help
Also called eighteenmo.

Origin of octodecimo

First recorded in 1855–60; short for New Latin in octōdecimō “in an eighteenth (of a sheet)”
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

Example sentences from the Web for octodecimo

  • Other sizes occasionally used are called “sixteenmo” or “sextodecimo,” “eighteenmo” or “octodecimo,” etc.

    The Story of Books|Gertrude Burford Rawlings

British Dictionary definitions for octodecimo

/ (ˌɒktəʊˈdɛsɪməʊ) /

noun plural -mos

bookbinding another word for eighteenmo

Word Origin for octodecimo

C18: from New Latin phrase in octodecimo in an eighteenth (of a whole sheet)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012