Each of the Five Chinese Elements (Wu Xing) has a Yang and a Yin component. These are known as the “Ten Heavenly Stems” (Tiān Gān 天干). The main importance of the Ten Stems has to do with the Chinese Calendar. Even though China adopted the Gregorian Calendar long ago, the traditional lunar Chinese calendar is still used for holidays and Astrology.
The Tiangan are Named as follows:
These Ten Stems occur in the above order in calendar Years. Coordinating with the Western Gregorian Calendar, Year Numbers ending in Four are Jia Years, Five are Yi, Six are Bing, Seven are Ding, Eight are Wu, Nine are Ji, Zero are Geng, One are Xin, Two are Ren, and Three are Gui. For example, 1924 was a Jia Year (Yang Wood), 1925 was a Yi Year, (Yin wood), and so on. Thus it can be seen, that each of the Five Elements in turn rules a period of Two Years at a Time. Please notice, that the Five Elements proceed through the Years in the arrangement known as the "mutual generation" (xiangsheng 相生) sequence.
The Ten Heavenly Stems combine with the Twelve Earthly Branches (Dìzhī 地支) to form a Sixty-Year (“Sexagenary”) Cycle. This will be discussed soon.