In precolumbian Aztec society, a calpulli (from Classical Nahuatl calpōlli, Nahuatl pronunciation: [kaɬˈpuːlːi], meaning "large house") was the designation of an organizational unit below the level of the altepetl "city-state". In Spanish sources, they are termed parcialidades or barrios. A Nahua city-state was divided into a number of calpullis, whose inhabitants were collectively responsible for different organizational and religious tasks in relation to the larger altepetl. Calpullis controlled land which was available for calpulli members to cultivate and also operated the Tēlpochcalli schools for young men of commoner descent. In Aztec culture, as in most other civilizations, the family unit was very important. There were several levels of organization in Aztec family life beginning with the base family unit. The base family unit consisted of two parents and their unmarried children. The main functions of the base family unit were education of the children and food preparation. Many base family units, however, banded together to form extended families. The households of extended families were usually composed of several brothers and their families. The primary functions of the extended families were to coordinate land use and food production (such as growing crops). In most cases, extended families contained just a few base family units. In large cities, however, they often grew to many more.
Although extended families farmed the land, they usually did not own it. They were allowed to use it by the calpulli to which they belonged. Calpulli were groups of families that controlled the use of the land and performed other territorial functions, as well as social ones. The majority of calpulli had a telpuchcalli, a school for young men. Another function of the calpulli was a taxation unit. The empire collected taxes from each calpulli, which in turn collected taxes from its member families.
Most rural calpulli were based on lineage. In other words, the members of a rural calpulli believed that they were descended from a common ancestor. In the cities, the calpulli were based more on geographical, political, and occupational similarities than lineage. In both cases, calpulli were tightly knit and sometimes were even somewhat isolated from other calpulli, both politically and physically. The nature of the interrelatedness between the members of the calpulli is a matter of debate. Traditionally it has been argued that the calpulli was firstly a family unit in which the inhabitants were related through blood and intermarriage. Other scholars such as Van Zantwijk (1985) deny that this was necessarily the case. Van Zantwijk demonstrates that at least in some altepetl the family-based nature of the calpulli was replaced with a hierarchical structure based on wealth and prestige. Newcomers from other altepetl were allowed to settle down and become part of the calpulli. Michael Smith (2003) shows that in some Nahua cities, notably Otumba, each calpulli specialised in a trade and took almost the form of a trade guild.
The wards of Tenochtitlan, the Aztec capital, were also known as calpulli. The exact number of calpullis within the city is not known but Van Zantwijk (1985) based his calculations on a wide array of ethnohistorical sources. Van Zantwijk reached a number of 20 calpullis, 7 of which represented the original founding groups of Tenochtitlan and the remaining 13 being composed by groups coming to the city at later points in its development. Van Zantwijk assumed that each calpulli was ritually connected to one of the twenty day names. Below is a list of the twenty calpullis of Tenochtitlan based on the data from Van Zantwijk (1985):