A more thorough explanation:

Definition: A codex is a book that is written on paper or parchment. It can also refer to a code, such as the Justinian Code.

Examples:

• The Codex Gigas is a medieval manuscript that is also known as the "Devil's Bible" because of its large size and mysterious illustrations.
• The Codex Justinianus is a collection of laws and legal interpretations that was created in the Byzantine Empire.

The examples illustrate the two different meanings of the term "codex." The Codex Gigas is an example of a book that was written on parchment, while the Codex Justinianus is an example of a code that was written on paper.