The Southern Plain. Surface Area: 483 km2. Cabeceras: Apasco, Hueypostla, Tequixquiac, Tezcatepec and Tuzantlalpa (twin cabeceras), Tlapanaloya.

The Southern Plain lies at the very northern end of the Valley of Mexico and would appear to fall naturally within its boundaries, but in fact this sub-area is part of the drainage system of the Tula River and is therefore included in the Valle del Mezquital. The decision to include the Southern Plain in the Valle del Mezquital on geographic criteria was reinforced by affiliation of the cabeceras within this plain with the preconquest geopolitical division known as the Teotlalpan.

Two subject pueblos of Hueypostla: Tlacotlapilco and Tezcatepec are located several kilometers from the cabecera in the North–South Plain and are not included in the Southern Plain; Tlilcuautla (subject to Tezcatepec and Tuzantlalpa), which lies at the eastern end of the Central Valley, was also excluded from the Southern Plain.

The North–South Plain. Surface Area: 753 km2. Cabeceras: Atitalaquia, Atotonilco, Chilcuautla, Mizquiaguala, Tezontepec, Tlacotlapilco, Tlahuelilpa, Tlamaco.

The main problem here was to separate the documentation for the pueblo of Tezcatepec located in the North–South Plain from that pertaining to the cabecera of the same name located in the Southern Plain. The documents pertaining to the northern pueblo were identified by references to its boundaries with the cabeceras of Mizquiaguala, Tezontepec, and Chilcuautla, and to its status as a subject of Hueypostla.