What is Heavenly Stems?

A cycle of ten characters (甲乙丙丁戊己庚辛壬癸) used in Chinese calendrical and cosmological systems. Each stem carries a Five Element and yin-yang polarity. In the najia system, Heavenly Stems are paired with Earthly Branches and assigned to hexagram lines. They also determine the "day master" in Bazi (Four Pillars) analysis.

Source: Jingshi Yizhuan (京氏易傳), Jing Fang