quar·to /ˈkwɔr(ˌ)to/
  四開,四開大的書本
  Quar·to a.  Having four leaves to the sheet; of the form or size of a quarto.
  Quar·to, n.; pl. Quartos  Originally, a book of the size of the fourth of sheet of printing paper; a size leaves; in present usage, a book of a square or nearly square form, and usually of large size.
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  quarto
       n : the size of a book whose pages are made by folding a sheet
           of paper twice to form four leaves [syn: 4to]