Twen`ty-four"mo (?), a.
Having twenty-four leaves to a sheet; as, a twenty-fourmo form, book, leaf, size, etc.
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n.
A book composed of sheets, each of which is folded into twenty-four leaves; hence, indicating more or less definitely a size of book whose sheets are so folded; -- usually written 24mo, or 24°.
© Webster 1913.