Simply put, a codex is a bound and printed book, which technically makes most, if not all, of the books on your shelves codices. However, this term is usually used in a very specialized way, to refer to ancient, medieval, and renaissance manuscripts. The study of these codices can yield very interesting information about the people who lived during these times, and they contain a variety of information from religious texts to accounts of historical events by people who lived through them. Many museums have a collection of codices which members of the public may be able to see on display.
This word comes from the Latin caudex, which means “tree trunk.” This word was also used to refer to the wooden tablets used to store temporary information, and when the Romans were introduced to the codex, they simply re-used the word, associating it with booklike objects which contained written information. You may also hear the word used in the modern sense to refer to books of laws and extensive compilations of information.
The codex appears to have been developed by the Greeks, in response to a papyrus shortage which caused them to turn to vellum, a form of paper made from sheepskin. Because vellum was time consuming and very expensive to produce, the Greeks started writing on both sides of the page and binding the vellum in a format which we would recognize as a book, supplanting the scroll, which had been the previous medium of storage for printed materials.
A true codex, therefore, has a binding which allows readers to open the book at any point, and it also has writing which covers both sides of the page. This is why ancient Asian books are not considered codices, although the Chinese probably developed bookbinding techniques before the Europeans. In early Asian books, the writing was only on one side of the page. The term “codex” is also sometimes used to refer to Mesoamerican manuscripts, despite the fact that they were not bound like traditional books.
It is not an exaggeration to say that the codex totally changed the usage of the written word. Before the development of the codex, people relied on scrolls and tablets to store information, and these methods were cumbersome and space consuming. The codex represented a compact and highly efficient way to store information, setting the stage for the dissemination of knowledge and the development of the printing press.