Chinese sexagenary cycle
The Chinese sexagenary cycle, also known as the Stems-and-Branches, is a cycle of sixty terms used for reckoning time. It appears as a means of recording days in the first Chinese written texts, the Shang oracle bones of the late second millennium BC. Its use to record years began around the middle of the 3rd century BC. The cycle and its variations have been an important part of the traditional calendrical systems in Chinese-influenced Asian states, particularly those of Japan, Korea, Taiwan and Vietnam.
This traditional method of numbering days and years no longer has any significant role in modern Chinese time keeping or the official calendar. However, the sexagenary cycle is still used in names of many historical events, like Chinese Xinhai Revolution and Japanese Boshin War. It also continues to have a role in contemporary Chinese astrology and fortune telling.
Each term in the sexagenary cycle consists of two Chinese characters, the first being one of the ten Heavenly Stems of the Shang-era week and the second being one of the twelve Earthly Branches representing the years of Jupiter's duodecennial orbital cycle. The first term jiǎzǐ () combines the first heavenly stem with the first earthly branch. The second term yǐchǒu () combines the second stem with the second branch. This pattern continues until both cycles conclude simultaneously with guǐhài (), after which it begins again at jiǎzǐ. This termination at ten and twelve's least common multiple leaves half of the combinations—such as jiǎchǒu (甲丑)—unused; this is traditionally explained by reference to pairing the stems and branches according to their yin and yang properties.