The Cihuacoatl or "Snake Woman" was a position that was always held by a man, and seems to have most important in the period around the conquest of Atzcapotzalco. This period saw a lot of internal upheavals in Tenochtitlan, which included the division of central authority between two men: The Tlatoani (who had hitherto been the supreme leader) and the Cihuacoatl.

This division probably reflects the religious conception of the Aztec universe: that the universe had been created from a male and a female principle. The fact the Cihuacoatl was a female name (it's also the name of a goddess presiding over childbirth and fertility) indicates that it was probably meant to represent the female principle.

The Chronicles relate that the Tlatoani took no decision without consulting the Snake Woman, and no decision was valid if Snake Woman had not given his consent. I've seen it suggested that the Cihuacoatl represented the internal affairs of the tribe, such as civil custom and religion, while the Tlatoani represented the tribe in its external affairs, like war and alliances, and as such had the higher significance for Spanish observers who saw the Tlatoani as the leader of the tribe.

How long this division of power between the two roles lasted isn't certain, as only one man is known to have exercised real power as the Cihuacoatl. This was Tlacaellel (1398- 1496), who in his eighty-odd years was the power behind at least three Tlatoanis. Before his appearance, hardly anything is heard of the Snake Woman; after his death there was such a leader but he seems to have been only one amongst many.