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Aztec society traditionally was divided into social classes. The Aztec social classes grew incredibly sophisticated and complex once the Mexican people settled and began to build their empire. It's been said that the class structure was so elaborate that it impressed the Spanish almost as much as the architecture of the empire.
The Mexica people, who later became the nucleus of the Aztec empire, were for a time a nomadic tribe looking for a home. As they moved south, they came into contact with advanced peoples. Many cultures of the day looked back to the impressive culture of the Toltecs, and the Aztecs came to admire the Toltec heritage. In fact, eventually the word for artistic creations would be toltecayotl, for the Toltecs, and the Aztecs themselves would claim to be descended from the great Toltec nobles.
The Mexicans were anxious to claim a Toltec heritage, so they chose a nobleman of Toltec origin as their first king, a man named Acamapichtli. He fathered a great many children by 20 wives, and his descendants became the heart of a new social class in the empire - the nobles or pipiltin (singular pilli). From then on, a king would always be chosen from among the pipiltin.
Basically, the ruling positions were not hereditary, but preference was given to those in the "royal families". Originally the pipiltin status was not hereditary, although the sons of pillis had access to better resources and education, so it was easier for them to become pillis. Later the class system took on hereditary aspects.
The nobles had many other privileges. They generally received a fuller education, they were allowed to wear fancier clothes and decorate their houses. They were allowed to hold important government offices. But not all had positions of authority - some were craftsmen, or even palace servants. Those who served with distinction could move up the ranks.
The second class were the macehualtin (people), originally peasants. Eduardo Noguera estimates that in later stages only 20% of the population was dedicated to agriculture and food production. The other 80% of society were warriors, artisans and traders.
Slaves or tlacotin also constituted an important class. Aztecs could become slaves because of debts, as a criminal punishment, or as war captives. A slave could have possessions and even own other slaves. Slavery in Aztec society was in some ways more humane than in Western cultures. While some slaves were punished criminals or prisoners of war, others sold themselves or their children into slavery due to economic hardship. Slaves could free themselves by repaying their purchase price. They could marry and own property, and their children were born free.
Traveling merchants called pochteca were a small, but important class as they not only facilitated commerce, but also communicated vital information across the empire and beyond its borders. They were often employed as spies.