The Maya used a base-20, or vigesimal, numeral system.
In a positional system, each place represents a higher power of the base. For the Maya, the positions generally progressed through 1, 20, 400, 8,000, and beyond. Their basic written numerals used a dot for one, a bar for five, and a shell-like sign for zero. Combining these symbols allowed them to express numbers efficiently.
The system was especially useful for recording dates, astronomical observations, tribute, and royal history. A dot in the second position represented one twenty, while a dot in the third represented four hundred. Unlike modern decimal notation, Maya numerals were usually arranged vertically, with higher place values above lower ones.
A common complication is the Maya calendar’s modified vigesimal structure. In Long Count dates, the third position represents 18 × 20, or 360, rather than 20 × 20, helping approximate the length of the solar year. This calendrical adjustment does not change the standard answer: the underlying numeral system is conventionally described as base 20.