The Aztecs used a flag to represent the number 20 in their writing. Their numerical system was vigesimal, meaning it was based on groups of 20 rather than groups of 10.
Dots represented quantities from 1 through 19, while a flag marked one complete group of 20. Several flags could be combined to show larger amounts, so four flags represented 80. A separate fir-tree-like sign represented 400, and an incense-bag symbol represented 8,000.
These signs appeared within the broader Aztec pictographic and logosyllabic writing system, which recorded names, places, tribute, dates, and historical events. The system was not an alphabet like modern English writing, and its symbols could carry both visual meanings and sound values.
A common mix-up is confusing the flag with the dot: dots counted individual units, but the flag indicated the important base-20 unit itself. This mathematical structure was also shared by the Maya, although the two cultures had distinct writing traditions.