On the Dayan Yarrow Stalk Method

I. The Philosophical Foundation of the "Great Expansion Number"

"The number of the Great Expansion is fifty; of these, forty-nine are used. Divide them into two to represent the two primal forces; suspend one to represent the three powers; count them by fours to represent the four seasons; return the remainders to represent the intercalary month; in five years there are two intercalations, therefore return the remainders again and then suspend." — Zhouyi: Xici I (Appended Utterances)

This passage of fewer than a hundred characters constructs the core syllabus of ancient Chinese numerology (Shushu).

1. Why "Fifty"$1

Regarding the origin of the "Great Expansion Number of Fifty," scholars throughout history have offered various views:

• The Sum of Heaven and Earth Numbers: The Xici states, "The number of Heaven is 25, the number of Earth is 30; together they make 55." The Song dynasty scholar Zhu Xi believed that 50 is the "Great Expansion Number" derived by subtracting 5 (the generative numbers of the Five Elements) from 55.
• The Center of the Hetu and Luoshu: In the Hetu, the center is 5 and 10, which multiply to 50. In the Luoshu, the center is 5, and the 10 of the five directions (eight directions plus center/up/down) also equals 50.
• The Pythagorean Theorem (Gougu): The sum of the squares of the sides of a 3-4-5 triangle ($9+16+25$) is exactly 50.

Regardless of the interpretation, "Fifty" symbolizes the "total capacity" and "fixed constant" of the universe.

2. Why is "Forty-nine Used"$2

In actual practice, one stalk must be removed from the fifty and respectfully returned to the tube or placed in front, untouched for the remainder of the process. This single stalk holds supreme status.

• The Dialectic of Essence and Function (Ti-Yong): This stalk represents "Taiji" (The Great Ultimate). Taiji is the source of the world; it is silent and motionless, yet it resonates and connects with everything. It is precisely because of the "stillness" of the Taiji that the "motion" of the remaining forty-nine stalks is possible.
• The Interplay of Void and Reality: This is a profound philosophical metaphor—Truth (Dao) is often hidden and non-manifest. What we can operate and calculate is the "Function" (Yong) manifested by the Dao, not the "Essence" (Ti) itself.