The sexagenary system uses 10 Heavenly Stems together with 12 Earthly Branches. Their repeating combinations form a 60-part cycle used in traditional East Asian calendrical notation.
The Heavenly Stems are an ancient Chinese sequence of ten ordered symbols. They were first attested in the Shang dynasty, when they were associated with days in a ten-day week and appeared in the names of Shang rulers and ancestors. Their later use expanded into calendars, astrology, timekeeping, and other systems of classification.
The 12 Earthly Branches provide the second sequence. Pairing the first Stem with the first Branch and advancing both lists produces 60 distinct pairings because 60 is the least common multiple of 10 and 12. After the sixtieth combination, the cycle starts again.
A frequent mistake is to answer 60, which is the length of the complete sexagenary cycle, not the number of Heavenly Stems. The Branches contribute 12 symbols, while the Stems contribute 10.