Five Stem Unions天干五合
The ten stems pair yin with yang to make five unions.
甲與己合，乙與庚合，丙與辛合，丁與壬合，戊與癸合。
Xie Ji Bian Fang Shu, vol. 1
What it is
Xie Ji Bian Fang Shu explains this with the River Chart, which pairs one with six and two with seven. Count the stems in order: the first, Jia, pairs with the sixth, Ji, and the second, Yi, with the seventh, Geng.
San Ming Tong Hui tells it as a marriage: Jia gives its sister Yi to Geng, so Yi and Geng unite. Each pair also turns into one element.
The five pairs
The five unions and the element each becomes.
- 甲己化土Becomes earth
- 乙庚化金Becomes metal
- 丙辛化水Becomes water
- 丁壬化木Becomes wood
- 戊癸化火Becomes fire
Sources San Ming Tong Hui, vol. 2; Xie Ji Bian Fang Shu, vols. 1 and 3.