The ruling line (卦主, "lord of the hexagram") is the line that sets the tone for the whole hexagram. Understand that line, and you understand a good part of the situation.
How to spot it
There are two common clues:
- The odd one out. When a single line differs from the other five, it usually leads. In The Army, the only yang line sits in the second place: the general among the troops. In Coming Back, the only yang line is at the very bottom: the light starting over.
- The fifth place. Among the line positions, line 5 is the ruler's place. In many hexagrams, such as Activity, the fifth line is the ruling one.
Two kinds
The idea already appears in Wang Bi, a third-century commentator. Later, the great Qing imperial edition, the Zhouyi zhezhong, separated two kinds: the line that constitutes the hexagram, giving it its character even from a weak place, and the line that governs it, having the virtue and position to lead. Sometimes they're the same line and sometimes not, and a hexagram can have more than one.
In your reading
If the ruling line comes up as a moving line, pay special attention: the change touches the very center of the matter. If it doesn't move, it can still help you see who or what holds the power in your situation.
Lists of ruling lines belong to the commentary tradition, not to the ancient text, and not every author agrees on them.