A trigram is a figure of three lines, each yin or yang. There are 2 × 2 × 2 = eight of them, the bāguà (八卦). Each has an image from nature and a quality:
- Heaven: creative strength.
- Earth: receptiveness.
- Thunder: what wakes things up and sets them moving.
- Water: danger, what flows and doesn't stop.
- Mountain: stillness.
- Wind: the gentle thing that gets in everywhere.
- Fire: clarity, what clings.
- Lake: joy, exchange.
Tradition also gives each one a place in a family (father, mother, three sons and three daughters), a part of the body and an animal.
Two trigrams, one hexagram
Every hexagram reads as one trigram below, the lower trigram, and one above, the upper trigram. Together they set a scene. In Water, Water is doubled, danger on top of danger. In Well-Being, Earth sits above Heaven, and the two meet.
Origins
Tradition says the sage Fu Xi drew the eight trigrams after watching the sky, the land and the animals. That's an origin legend, not a historical record. Many scholars think reading hexagrams as pairs of trigrams developed over time, and we see it fully formed in the commentaries of the Ten Wings.
There are two classic ways to arrange them in a circle: the Earlier Heaven and the Later Heaven. You can go further in The Eight Trigrams.