Sheep to Sheet (of Parchment)

The question is: why are medieval books so big? The answer, courtesy of the Got Medieval blog: "medieval books were the size they were because medieval sheep were the size they were." The essay includes an instructive set of photos depicting sheep to sheet, and then all the folds that are made to create book sizes -- folio, quarto, octavo, sixteenmo, and the mini thirty-twomo. Pictured here is a page from the medieval manuscript Book of Kells (folio).