and regional schools According to Donald Robertson, the first scholar to attempt a systematic classification of Aztec pictorial manuscripts, the pre-Conquest style of Mesoamerican pictorials in Central Mexico can be defined as being similar to that of the Mixtec. This has historical reasons, for according to Codex Xolotl and historians like Ixtlilxochitl, the art of tlacuilolli or manuscript painting was introduced to the Tolteca-Chichimeca ancestors of the Tetzcocans by the Tlaoilolaques and Chimalpanecas, two Toltec tribes from the lands of the Mixtecs. The Mixtec style would be defined by the usage of the native "frame line", which has the primary purpose of enclosing areas of color. as well as to qualify symbolically areas thus enclosed. Colour is usually applied within such linear boundaries, without any modeling or shading. Human forms can be divided into separable, component parts, while architectural forms are not realistic, but bound by conventions. Tridimensionality and perspective is absent. In contrast, post-Conquest codices present the use of European contour lines varying in width, and illusions of tridimensionality and perspective. Later on, Elizabeth Hill-Boone gave a more precise definition of the Aztec pictorial style, suggesting the existence of a particular Aztec style as a variant of the Mixteca-Puebla style, characterized by more naturalism and the use of particular calendrical glyphs that are slightly different from those of the Mixtec codices. Regarding local schools within the Aztec pictorial style, Robertson was the first to distinguish three of them: School of Mexico Tenochtitlan: Based at the imperial capital of Tenochtitlan, it comprises two stages, an early one which would include the Matrícula de Tributos, Plano en Papel de Maguey, Codex Boturini and the Codex Borgia; and a later one, which would comprise Codex Mendoza, Codex Telleriano-Remensis, Codex Osuna, Codex Mexicanus and the Magliabechiano Group. School of Texcoco: Based at the Texcoco polity (altepetl), this school comprises documents related to the court of Nezahualcoyotl. Its foremost representativese are the Mapa Quinatzin, Mapa Tlotzin, Codex Xolotl, Codex en Cruz, the Boban Calendar Wheel, and the Relaciones Geográficas de Texcoco. School of Tlatelolco: Based at the sister-city of Tenochtitlan, Tlatelolco, this school is associated with the Badianus herbal, the Mapa de Santa Cruz, the Codex of Tlatelolco and the Florentine Codex. Survival and preservation A large number of prehispanic and colonial indigenous texts have been destroyed or lost over time. For example, when Hernan Cortés and his six hundred conquistadores landed on the