The transition from the papyrus scroll, the preferred medium of ancient writing, to the parchment codex, with separate pages sewn together (a book), would transform the library. … Yet parchment was not widely used in the ancient world, relied upon only during shortages of papyrus. In an era of abundant and cheap papyrus, this is understandable: parchment took longer to prepare and was far more expensive: a substantial parchment book would require the skins of more than a hundred calves, goats or sheep. But it did have two advantages: it was durable and would not degrade in climates colder than that of the Egyptian Nile delta, and one could write on both sides of the sheet, instead of the single side of papyrus.

Andrew Pettegree, Arthur der Weduwen, The Library- A Fragile Hist…, loc. 84