(1) Sheet folded twice, making pages one-fourth the size of the original sheet. A quarto makes an 8-page signature. (2) Book made from quarto sheets, traditionally measuring about 9&#39; x 12&#39;.<br><br>A book or manuscript of the second largest standard size, usually measuring 32 cm (12 1/2 inches) in height, and 24 cm (9 1/2 inches) in width, which is composed of sheets of paper folded into four leaves. The plural form is quartos. Abbreviated q., Q., qto, Qto, and 4to.Also see bookbinding, duodecimo, folio, octavo, rotulus, sextodecimo, signature, text, tricesimo-segundo, and vicesimo-quarto.