Vigesimal

The Maya numerals are an example of a base-20 numeral system.
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A vigesimal (/vɪˈdʒɛsɪməl/ vij-ESS-im-əl) or base-20 (base-score) numeral system is based on twenty (in the same way in which the decimal numeral system is based on ten). Vigesimal is derived from the Latin adjective vicesimus, meaning 'twentieth'.