A book size or format where the sheet is folded 16 times, resulting in pages that are one-sixteenth the size of the original sheet.
From Italian 'sedicesimo' and Spanish 'dieciséis' (sixteenth), entering English through printing terminology. 'Deci-' relates to ten, 'sexto' to six, combined to mean sixteen-fold.
Decimosexto books are incredibly rare because they're impractically tiny—imagine trying to read a book the size of your palm with regular eyesight—which is why they mostly exist as curiosities in rare book collections.
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