During the Classic period the linguistic situation simultaneously becomes both clearer and more obscure. While the Maya actually left examples of their writing, researchers have been unable to determine the linguistic affiliations of several important Classic civilizations, including Teotihuacan, Xochicalco, Cacaxtla, and El Tajín. During this time it is well established that Mixtec languages were spoken at Tilantongo and Zapotec at Monte Albán (in the Valley of Oaxaca). The linguistic situation of the Maya area is relatively clear – Proto-Yucatec and Proto-Cholan were established in their respective locations in Yucatán and in the Tabasco area. Around 200 CE speakers of the Tzeltalan branch of Proto-Cholan moved south into Chiapas displacing speakers of Zoquean languages. Throughout the southern part of the Maya area and the highlands the elite of the Classic Maya centers spoke a common prestige language based on Cholan, a variant often referred to as Classic Ch'olti'an.