Tian Gan (天干) refers to the ten Heavenly Stems in the system of Tian Gan Di Zhi (天干地支), a traditional Chinese cyclical notation system. They are also known by other names such as "Yan Feng (阏逢), Zhan Meng (旃蒙), Rou Zhao (柔兆), Qiang Yu (强圉), Zhuo Yong (著雍), Tu Wei (屠维), Shang Zhang (上章), Chong Guang (重光), Xuan Yi (玄黓), Zhao Yang (昭阳)." The five elements of the Heavenly Stems are divided into Yin and Yang. Specifically, they are: Jia Mu (甲木), Yi Mu (乙木), Bing Huo (丙火), Ding Huo (丁火), Wu Tu (戊土), Ji Tu (己土), Geng Jin (庚金), Xin Jin (辛金), Ren Shui (壬水), Gui Shui (癸水), where Jia, Bing, Wu, Geng, and Ren are Yang, and Yi, Ding, Ji, Xin, and Gui are Yin. In the Chinese calendar, Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, and Gui are called the "Ten Heavenly Stems," while Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, and Hai are called the "Twelve Earthly Branches." These two are combined in a fixed order to form the Gan Zhi (干支) chronological system. According to the "Ciyuan (辞源)," the terms Gan and Zhi derive their meanings from the "trunk and branches" of trees. The Tian Gan Di Zhi system forms a unique Chinese calendar used to record years, months, days, and hours. In the "I Ching (易经)" and traditional Chinese medicine systems, the Heavenly Stems correspond to various related information, such as geographical directions, the five internal organs, and other analogies of all things.