The eight trigrams and the sixty-four hexagrams: how a hexagram is built
Sixty-four hexagrams look like a lot, but they are built in only two layers: first eight small figures of three lines each, then these stacked two by two. Once the structure is clear, every hexagram name has a reason.
The eight small figures and their pictures
The basic unit is the trigram, made of three lines, each with a picture and a temperament. The Discussion of the Trigrams puts it like this:
Qian, Heaven: strong and untiring.
Kun, Earth: soft and yielding.
Zhen, Thunder: arousing, rising up.
Xun, Wind: penetrating, gentle.
Kan, Water: sinking, danger.
Li, Fire: clinging, bright.
Gen, Mountain: stillness, stopping.
Dui, Lake: joy, an open mouth.
Eight times eight is sixty-four
Put one trigram below and another above, and you have a hexagram of six lines; there are 8×8=64 of them. The names come the same way: Tai (Earth over Heaven) is Kun (Earth) above and Qian (Heaven) below; Zhun (Water over Thunder) is Kan (Water) above and Zhen (Thunder) below.
So when you read a hexagram, first picture the two images together: a spring at the foot of a mountain is Meng; water in the sky is Xu; thunder under the earth is Fu. The picture is the best way into a hexagram.
Reversed and inverted hexagrams: hexagrams in pairs
The sixty-four are not isolated. Turn all six lines of a hexagram over, yin to yang and yang to yin, and you get its reversed hexagram, for example Tai and Pi. Turn a hexagram upside down and you get its inverted hexagram, for example Zhun and Meng.
Eight hexagrams stay themselves when turned upside down (Qian, Kun, Yi, Da Guo, Kan, Li, Zhong Fu, Xiao Guo), and these pair with their reversed hexagram. The other fifty-six are inverted pairs of each other. The order of the Book of Changes goes down in pairs, so when you read one hexagram, a glance at its partner often shows something new.
The upper and the lower canon
By tradition the sixty-four hexagrams divide into the upper canon of thirty and the lower canon of thirty-four. The upper begins with Qian and Kun and ends with Kan and Li; the lower begins with Xian and Heng and ends with Ji Ji and Wei Ji. The sixty-third, Ji Ji, is "already crossed", and the last, the sixty-fourth, Wei Ji, is "not yet crossed": ending on a hexagram with no ending means that things always go on.
On every hexagram page of this site you will see its number and which canon it belongs to, and its reversed and inverted partners, so you can read on pair by pair.
Questions people ask
Why is the order not the binary order of yin and yang?
The received order of the Book of Changes runs in pairs, each pair setting two situations against each other. Shao Yong of the Song dynasty drew up a binary "Fuxi order of the sixty-four hexagrams", but that is a different arrangement. This site uses the received order.
What do the trigrams have to do with feng shui and fortune-telling?
Later feng shui and fate-reading borrow the pictures of the trigrams, but they are separate systems with different uses. What is described here is the structure of the Book of Changes itself, used only as a tool for reading its words.