The numbering and math system we commonly use is the base-10, or decimal system. But if you remember from your school days, math principles still work on any system. Base-20 is called the vigesimal system, from the Latin “vigesimus,” with “viginti” meaning 20. The ancient Maya and Aztec civilizations used vigesimal numbers, and the modern French numbering system has evidence of a past vigesimal system (the word for 80 is “quatre-vingts,” or “four-twenties”). We won’t get too deep into how a vigesimal system works, but instead of things being structured around 10s, 100s, 1,000s, and so on, the math works around 20s. As a regular adjective (outside of the math system), “vigesimal” can apply to anything related to 20, and it can be a synonym for 20th.