The size of a volume created by folding a sheet in half once, then rotated 90 degrees and folded in half again. This creates a quire of four leaves (eight pages) that is one-quarter the size of the original sheet. Abbreviated 4o or 4to.
tv , Michelle Brown
En cuarto; In-quarto
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).