According to traditions recorded in several of the post-conquest historical sources, Xólotl, a 12th-century ruler of a Chichimec polity in the Valley of Mexico, ordered a review to be undertaken to enumerate the populace under his control.[2] The retelling of this tradition was documented in the late 18th century by Francesco Clavigero, based on Fray Juan de Torquemada's Monarchia Indiana, first published in 1615.[4] During the later Aztec Empire, it is known that written census-like records were used to keep track of land ownership and the tribute obligations of individual city-states (altepetl) across central Mexico.[5] In the decades after the conquest and Spanish colonial expansion, the administrators and missionaries for the Real Audiencia of Mexico began the systematic collection of population data for the new territories.Conducted under viceroy Juan de Güemes Padilla, Count of Revillagigedo between 1790 and 1791, some forty volumes of data from this census are conserved in the Mexican national archives.