I Ching Line Positions Explained: Proper Place, Center, Correspondence, Adjacency, Supporting and Riding

Yang lines belong in places 1, 3, 5, yin lines in 2, 4, 6; places 2 and 5 are central; 1-4, 2-5 and 3-6 correspond. Plus supporting, riding and adjacent lines.

The six places

A hexagram has six lines, counted from the bottom up: first, second, third, fourth, fifth and top. The first, third and fifth are odd-numbered places, and these are yang places; the second, fourth and top are even-numbered places, and these are yin places. The lower three lines form the lower (inner) trigram, and the upper three lines form the upper (outer) trigram.

The second and the fourth lines have the same function but different places, and they do not fare alike: the second is mostly praised, the fourth is mostly fearful, because it is near. ... The third and the fifth lines have the same function but different places: the third mostly meets misfortune, the fifth mostly achieves merit. Such is the difference between high rank and low.

This comes from the second part of the Xici (the "Great Commentary"). The second and fifth lines hold the center of the lower and upper trigrams, and they mostly meet good fortune. The third line is at the very top of the lower trigram, about to move up but not yet there, and it mostly meets misfortune. The fourth line presses close to the fifth, the place of honor, and it is mostly fearful. The first line is the beginning of a matter; the top line is its end, where things reach their limit and change.

Proper place and the center

Proper place (dangwei; also called dezheng, "being correct"): a yang line in a yang place, or a yin line in a yin place. In Jiji (After Completion) all six lines are in their proper places; in Weiji (Before Completion) none of them is.
Central (dezhong): standing in the second or the fifth place. Being central matters more than being correct. A line that is not in its proper place but is central is often still fortunate; a line that is in its proper place but not central is not necessarily fortunate.
Central and correct (zhongzheng): both at once, which means a Six in the second place or a Nine in the fifth place. This is the best of all.

Correspondence and adjacency

Correspondence (ying): the first and fourth lines, the second and fifth, and the third and top hold matching places in the two trigrams. When one is yin and the other yang, there is correspondence: the two answer each other, and each has support. When both are yin or both are yang, there is no correspondence (this is also called hostile correspondence).
Adjacency (bi): the relation between two neighboring lines. Here too, one yin and one yang makes for a close bond.
Supporting (cheng, 承): a yin line beneath a yang line. The soft supports the firm from below; this goes with the grain.
Riding (cheng, 乘): a yin line above a yang line. The soft rides on the firm; this goes against the grain and is mostly unfortunate.

How to use this

These are not formulas for calculating good fortune and misfortune. They are the language that the Tuanzhuan ("Commentary on the Judgments") and the Xiaoxiang ("Lesser Image" commentary on the lines) use when they explain the line statements. For example, the Nine in the fifth place of Qian (Heaven) is a yang line in a yang place and also holds the center of the upper trigram. It is central and correct, and so: "A dragon flying in the sky." The Six in the third place of Kun (Exhaustion) is a yin line in a yang place, it rides on a firm line, and it has no correspondence, and so: "Hemmed in by stone, leaning on thorns."

On this site, the page for each line lists whether it is in its proper place, whether it is central, and its correspondence.