Codex is the formal name for what we commonly call a book. With sequential pages bound together on one side, the codex is a compact sturdy way of storing and accessing the printed word. The pages can be opened flat for ease of reading. It is economical since both sides of the page can be used. And the binding protects the content. It is not surprising that the codex replaced the scroll as the preferred format for the book. Or that e-books emulate the traditional codex.