A codex is an ancient manuscript in book form, consisting of pages made from materials like parchment or papyrus. Unlike scrolls, which were rolled up, codices allowed for easier access to information, as readers could flip through pages. This format became popular in the early centuries of the Common Era, especially among early Christians. The transition from scrolls to codices marked a significant development in the history of writing and literature. Notable examples of codices include the Codex Sinaiticus, an important early Christian Bible, and the Codex Mendoza, which provides insights into the Aztec civilization.