dating methods, and a certain degree of self-interest, create difficulties in establishing priority and thus the forebear from which the others developed. The best documented and deciphered Mesoamerican writing system, and therefore the most widely known, is the classic Maya script. Others include the Olmec, Zapotec, and Epi-Olmec/Isthmian writing systems. An extensive Mesoamerican literature has been conserved partly in indigenous scripts and partly in the postinvasion transcriptions into Latin script. The other glyphic writing systems of Mesoamerica, and their interpretation, have been subject to much debate. One important ongoing discussion regards whether non-Maya Mesoamerican texts can be considered examples of true writing or whether non-Maya Mesoamerican texts are best understood as pictographic conventions that express ideas, specifically religious ones, but don't represent the phonetics of spoken language. Mesoamerican writing is found in several mediums, including large stone monuments such as stelae, carved directly onto architecture, carved or painted over stucco (e.g., murals), and on pottery. No Precolumbian Mesoamerican society is known to have had widespread literacy, and literacy was probably restricted to particular social classes, including scribes, painters, merchants, and the nobility. The Mesoamerican book was typically written with brush and colored inks on a paper prepared from the inner bark of the ficus amacus. The book consisted of a long strip of the prepared bark, which was folded like a screenfold to define individual pages. The pages were often covered and protected by elaborately carved book boards. Some books were composed of square pages while others were composed of rectangular pages. Following the Spanish conquests in the sixteenth century, Spanish friars taught indigenous scribes to write their languages in alphabetic texts. Many oral histories of the prehispanic period were subsequently recorded in alphabetic texts. The indigenous in central and southern Mexico continued to produce written texts in the colonial period, many with pictorial elements. An important scholarly reference work is the Handbook of Middle American Indians, Guide to Ethnohistorical Sources. Mesoamerican codices survive from the Aztec, Maya, Mixtec, and Zapotec regions. Arithmetic Mesoamerican arithmetic treated numbers as having both literal and symbolic value, the result of the dualistic nature that characterized Mesoamerican ideology. As mentioned, the Mesoamerican numbering system was vigesimal (i.e., based on the number 20). In representing numbers, a series of bars and dots were employed. Dots had a value of one, and bars had a value of five. This type of arithmetic was combined with a