The origin of the Mixtec writing system is unknown, but other Mesoamerican pre-Columbian writing systems, such as that of the Aztecs, are found to have similar traits. The Mixtec writing system is found on codices dating to the Post-Classic period preceding the arrival of the Europeans in 1520 AD. The Mixtec codices are writings recorded on decorated strips of bark and animal hide. The term codex (singular) is usually applied to bound books, though scholars use it to refer to Mesoamerican writings. The codices are constructed by folding bark over to form an accordion-like book, or by covering strips of bark and leather with plaster to form a smooth writing surface.