The tonalpohualli pairs twenty named day signs, Crocodile, Wind, House, Lizard, Serpent, Death, Deer, Rabbit, Water, Dog, Monkey, Grass, Reed, Jaguar, Eagle, Vulture, Movement, Flint, Rain and Flower among them, with the numbers one through thirteen running independently alongside them, so a full combination of sign and number repeats only after two hundred sixty days, since twenty and thirteen share no common factor smaller than their own product. The cycle divides into twenty trecenas, thirteen day spans each opened by day one of a particular sign, and each trecena was held to fall under the patronage of a specific deity pair whose favor coloured the fortunes of everyone born within it.
The tonalamatl, literally "book of days," was the painted almanac priests called tonalpouhque consulted to read a child's day sign at birth and forecast the character and fate that sign carried, to choose auspicious dates for marriages, journeys and warfare, and to time the great state rituals against the day on which they fell. The two hundred sixty day count has no straightforward tie to any single astronomical period, and its true origin is uncertain. What is well attested is that the same cycle, under different sign names, structured the Maya tzolkin and calendars across Mesoamerica well before either the Aztec or the Maya state existed in the form the Spanish encountered.