Variant or alternative term for octavo, a book size created by folding a sheet of paper three times to produce eight leaves (sixteen pages).
From Latin 'in octavo' (in eights), written phonetically as 'eightvo' in English. An alternative spelling emphasizing the English pronunciation of the Latin term.
Different languages developed different names for the same book sizes—'eightvo' and 'octavo' mean identical things, showing how English and Latin printing terminology got tangled together!
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