Si is the sixth Earthly Branch, belonging to Yin Fire, associated with the Snake zodiac sign, and corresponds to the Si Hour (09:00–11:00). Si Fire represents early-summer fire, internally concealing Bing Fire, Wu Earth, and Geng Metal; its nature is warm yet reserved, symbolizing wisdom, strategy, and transformative power. Si Fire embodies the image of the snake—skilled in strategy and adept at concealing its true intentions.
Si Fire is identified from the Earthly Branches of the Four Pillars in a Bazi chart. If Si appears in the Year Branch, the native belongs to the Snake zodiac; if Si appears in the Day Branch, the Spouse Palace is governed by Si Fire. Si Fire is strongest during the Si Month (lunar fourth month), when it is in season.
A person born on a Gui-Si Day has Si Fire as Direct Wealth in the Day Branch—suggesting a spouse skilled in financial management and possessing a reserved disposition.