The Aztecs formed an extensive state (often referred to as the Aztec Empire) in pre-Columbian Mesoamerica, based in the central Mexican highlands during the late Postclassic period in Mesoamerican chronology. Since its popularization in historical literature from the 19th century onwards, the term Aztec has been used to refer to several different (but related) groupings of Nahuatl-speaking peoples, and its usage varies somewhat with the context. Often, it refers exclusively to the people who traditionally founded their capital Tenochtitlan, who were self-described as the Mexica, Culhua-Mexica or Tenochca. Sometimes it also includes Tenochtitlan's two principal city-state allies, the Acolhuas of Texcoco and the Tepanecs of Tlacopan, who together with Tenochtitlan formed the Aztec Triple Alliance that gave rise to the extensive empire in the 14th, 15th and 16th centuries. In other contexts it may refer to all the various Nahua peoples and altepetls that became participants and tributaries to the dominating state.