The codex was the historical ancestor of the modern book. Instead of being composed of sheets of paper, it used sheets of vellum, papyrus, or other materials. The term codex is often used for ancient manuscript books, with handwritten contents. A codex is bound by stacking the pages and securing one set of edges, in a form analogous to modern bookbinding. Modern books are divided into paperback and those bound with stiff boards, called hardbacks. Elaborate historical bindings are called treasure bindings. At least in the Western world, the main alternative to the paged codex format for a long document was the continuous scroll, which was the dominant form of document in the ancient world. Some codices are continuously folded like a concertina, in particular the Maya codices and Aztec codices, which are actually long sheets of paper or animal skin folded into pages. In Japan, concertina-style codices called orihon developed during the Heian period (794–1185) were made of paper.