A codex (the plural is codices) is the name bibliographers give to the format of virtually all modern books: a series of sheets folded and bound together so as to produce separate pages. The codex first appeared around the fourth century C.E., and offered several advantages over the scroll, not least that it was possible to jump easily from one part of a book to another. A scroll pretty much has to be read sequentially: if you want to get from the beginning to the end, you have to unroll the whole thing. A codex, however, lets you turn from any one page to any other with minimal problems. It made possible things like page numbers and indexes, and helped to produce a revolution in the way the written word was presented.

Note that the codex book long preceded the invention of printing. For more than the first thousand years of its history, codices were used for manuscripts, not printed books.

From the Guide to Literary Terms by Jack Lynch.
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