The page dimensions, 4 by 6 1/2 inches, of a book consists of printer's sheets folded into 18 leaves or 36 pages.
A book consists of octodecimo pages.
A size of a sheet of report resulting from folding and cutting a sheet of paper into eighteenths (3.5"-4.5" x 4.9"-5.9").
a novel composed of pages of the dimensions.
a novel made up of sheets each of which can be collapsed into eighteen leaves; therefore; indicating more or less absolutely a size of guide, whoever sheets are creased; -- typically written 18mo or 18°, and called eighteenmo.