Introduction to Chen, Xu, Chou, Wei and the Wu/Ji Earth Stems
In Bazi, four Earthly Branches — Chen (辰), Xu (戌), Chou (丑), and Wei (未) — are known as the Si Ku (四庫), the Four Vaults. Each one carries a Wu (戊) or Ji (己) Earth Heavenly Stem as its dominant hidden stem, which is exactly why these four branches are the strongest root a Wu or Ji Earth Day Master can have.
What Is the Relationship Between the Four Vaults and Wu/Ji Earth?
Chen and Xu store Wu Earth (戊) as their chief hidden stem; Chou and Wei store Ji Earth (己). That single fact splits the four vault branches into two pairs — a Yang-Earth pair and a Yin-Earth pair — and it decides which branches actually root a given Earth Day Master.
The Four Vault Branches, One by One
Chen (辰) — The Dragon, Root of Yang Earth
Chen's hidden stems are Wu (戊, chief), Yi (乙, middle), and Gui (癸, residual). It stores the tail end of Water and a trace of Wood inside a Yang-Earth shell — a "wet earth" branch that roots Wu Earth while quietly feeding Water and Wood influences into the chart.
Xu (戌) — The Dog, Root of Yang Earth
Xu's hidden stems are Wu (戊, chief), Xin (辛, middle), and Ding (丁, residual). It stores Metal and Fire inside a "dry earth" shell, so a Wu Day Master rooted in Xu tends to run drier and firmer than one rooted in Chen.
Chou (丑) — The Ox, Root of Yin Earth
Chou's hidden stems are Ji (己, chief), Gui (癸, middle), and Xin (辛, residual). Cold, wet Earth that stores Water and Metal — the classic root for a Ji Earth Day Master born in winter.
Wei (未) — The Goat, Root of Yin Earth
Wei's hidden stems are Ji (己, chief), Ding (丁, middle), and Yi (乙, residual). Warm, dry Earth that stores Fire and Wood — the summer counterpart to Chou.
How This Affects a Wu or Ji Day Master
A Wu Earth Day Master is considered rooted when Chen or Xu appears anywhere in the chart or the current Luck Pillar; a Ji Earth Day Master looks to Chou or Wei instead. A rootless Earth Day Master reads as weak no matter how many Earth-generating stems surround it, so check the branches before judging Day Master strength — use the free Bazi calculator to lay out your own four pillars and confirm which vault branches you actually have.
Clashes and Punishments Among the Four Vaults
The four vaults don't sit quietly next to each other. Chen and Xu clash directly (辰戌相沖), and so do Chou and Wei (丑未相沖) — each clash breaks the vault open and releases its stored elements into active use, for better or worse depending on what the chart needs. Separately, Chou, Xu, and Wei form the Chou-Wei-Xu three-branch punishment (丑未戌三刑), while Chen instead carries its own self-punishment (辰刑辰) rather than joining that triangle. For the full harmony, combination, clash, and punishment reference across all twelve branches, see the Twelve Earthly Branches guide.
Key Takeaways
- Chen (辰) and Xu (戌) are the strongest roots for a Wu Earth (戊) Day Master; Chou (丑) and Wei (未) are the strongest roots for a Ji Earth (己) Day Master.
- Each vault branch also stores a second and third hidden stem — Water/Wood in Chen, Metal/Fire in Xu, Water/Metal in Chou, Fire/Wood in Wei.
- Chen-Xu and Chou-Wei each form a direct clash, releasing the vault's stored elements.
- Chou, Xu, and Wei together form the three-branch punishment; Chen instead carries a self-punishment.
- Use the free Bazi calculator to see which vault branches your own chart carries before judging Day Master strength.
