A codex is the form that most books take today: paper is cut up into leaves and bound between two covers. Each leaf contains two pages of writing – one on each side. Invented in the late first century, it replaced the scroll in the third and fourth centuries.
To us a codex is simply a book, but when they first appeared it was called a codex. Back then when you said “book”, people thought of a scroll. Jesus did not open a book – he unrolled it!
Because it uses both sides of the paper, a codex requires only half as much paper as a scroll. It could also fit more paper into a smaller space. This meant that even an early codex could replace a box of four or five scrolls. Because of the covers, the paper could be much thinner than in a scroll, so over time the difference became even greater. All this made a codex much easier to carry.
Not just easier to carry, but also easier to use: to go from one page to another you did not have to go through all the pages in between. So you could go to a given page much more quickly. This made it easier to follow a line of argument and to do research. You could bookmark several places in the book and move quickly between them. Scrolls had bookmarks too, but moving between them was a pain.
The codex is such a good invention that even in this age of invention it still holds its ground. E-books, for example, have made almost no headway against the codex.
And yet even so, it took over 300 years for the codex to replace the scroll. And even then it may have succeeded less on its merits and more because of a change in religion in the Roman Empire: Christians preferred the codex over the scroll and the codex did not become the preferred form till most people were Christians! The four gospels required four scrolls but only one codex.
The codex was invented at least twice. It was invented a second time in the 1100s in Japan as the butterfly book. Because the ink they used went through paper, however, only one side of a leaf had writing on it, so it did not have the advantages of the Western codex.
Both the Japanese and the Mayans invented a book form halfway between the scroll and the codex: the folded book. Instead of rolling the paper to make a scroll, it was folded between each page to fold up into something very similar to a codex. The Japanese called it an orihon, a folding book, and even added covers.