Two words for one hillside
Before they were philosophy, 陰 yin and 陽 yang were words about the landscape. Yin was the side of a hill in shade; yang, the side facing the sun. It's the same hill seen at two times of day. That plain image already holds the essence: they aren't two separate things, but two faces of something that keeps changing.
Over time, yin and yang became the classical Chinese way of thinking about any complementary pair: what opens and what closes, what pushes forward and what receives, day and night, activity and rest.
Yin and yang in the I Ching
In the I Ching, yin and yang are written as lines:
Three lines make the eight trigrams, and six make the 64 hexagrams. The whole system is built from these two pieces. Hexagram 1, Activity, is pure yang, and hexagram 2, Passivity, is pure yin. Every mix lies between them.
Here's something surprising: in the oldest layer, the Zhouyi, the words yin and yang hardly appear. The lines were already there, but the language of yin and yang used to explain them comes mostly from the later commentaries, the Ten Wings. One of them puts it this way: 一陰一陽之謂道, "one yin, one yang: that is called the Way." The same text tells how everything begins: 易有太極，是生兩儀，兩儀生四象，四象生八卦, "change has a supreme origin; it gives birth to the two forms, the two forms give birth to the four images, and the four images to the eight trigrams."
Old and young: why lines change
Those "four images" show up in the values you get when you cast: 6, 7, 8 and 9.
- 7: young yang. Solid and settled.
- 8: young yin. Open and settled.
- 9: old yang. It has reached its peak and is about to turn yin.
- 6: old yin. It has reached its peak and is about to turn yang.
The old values are the moving lines. The idea behind them is pure I Ching: when something reaches its extreme, it starts turning into its opposite. Noon already holds the afternoon, and deep winter holds the spring. Moving Lines covers this in detail.
Common misconceptions
"Yang is good and yin is bad." No. Neither is better. A hexagram full of yin isn't a bad sign, and a yang line isn't a "yes." In the I Ching the question isn't which one you have, but whether each is in its place and its time.
"Yang is male and yin is female." Tradition does use that association, for example in the trigram family, where Heaven is the father and Earth the mother. But it's one image among many, not a rule about people. Everyone has times to take the lead and times to receive, times to speak and times to listen.
"They're fixed." Quite the opposite. Yin and yang are defined by their relationship. Something is yin compared with something else. A mountain is yang next to a valley and yin next to the sky. And each keeps turning into the other, which is why this is a book of changes.
"More yang always means more strength." Not always. The Great Commentary (繫辭) points out something odd: the trigrams tradition calls yang, like Thunder or Water, have just one yang line between two yin. The line in the minority is the one in charge. A little, well placed, outweighs a lot.
Using it when you read
When you look at a hexagram, notice the balance. Are there more solid lines or more open ones? Where are they? A single yang line on top of five yin, as in hexagram 23, The Fall, tells a very different story from a single yang line at the bottom, as in hexagram 24, Coming Back. You don't need to be an expert: just looking at the shape tells you a lot.
To see how these lines combine in threes, read The Eight Trigrams.