What exactly was a ‘calpulli’?

Good question, Stanly!
Calpulli in Náhuatl means ‘great house’ or ‘group of houses’. An important - yet mysterious - unit of Mexica social organisation, it has been variously defined as neighbourhood (barrio in Spanish), district, clan, tribe, town, community, parish, village ward, agriculture-based cooperative, ‘group of familiies who lived near one another’, ‘group of households forming a small barrio and having common tributary obligations’... Considerable argument and uncertainty surround its true meaning and internal workings. About the only thing we CAN be sure of is that EVERY Aztec/Mexica citizen identified with their local calpulli (sometimes written calpolli). As a building-block of Aztec society, it survived well into the colonial period.

Pic 1: Illustration by Professor Michael E Smith of a rural noble’s compound (A) compared to a commoner’s house (B)

More recently, scholars have expressed doubt over just how strong the kinship/descent principle was among the Mexica - in some calpultin (plural of calpulli) land was found not to be automatically passed down to the next generation - leading some to abandon the idea of these groupings as ‘clans’. In rural areas calpultin became generally less powerful than their urban counterparts, often being spread out over wide geographical areas and subjected to the semi-feudal power of noble lords (see pic 1): they ranged hugely from small clusters of just 10-20 households (together with the land assigned to them) to far larger units; Michael Smith found one calpulli at Molotlan consisting of ‘128 households, divided into 9 wards ranging from 1-32 households in each’.
Urban calpultin, by contrast, were more tightly knit, often grouping together and based around common occupations (such as merchants) or craft specialities such as featherworkers, organised, like those of Amantlan mentioned in the Florentine Codex (see pic 4) into guilds.

Pic 3: The ‘constant round of activities’ of the calpulli councils of elders may have proved ‘crucial for the orchestration of the glamour of local identity’

Whilst the importance of the calpulli may have diminished somewhat as the imperial power of Tenochtitlan grew, there’s no doubt that it continued to provide a key focus for pride and identity among Mexica citizenry. This is reflected in the changing role of the calpulli council of elders: however symbolic this may have become - particularly under Moctezuma II - in the words of Inga Clendinnen ‘I suspect that their constant round of activities - welcoming the new crop of infants into the calpulli at the local temple, “blessing” feasts, sanctioning marriages, honouring returning warriors and singing their triumphs - was crucial for the orchestration of the glamour of local identity.’

Good luck with your novel, Stanly!

Pic 5: Each calpulli gave families and individuals a unique source of pride; figurine, Diego Rivera Anahuacalli Museum, Mexico City