I Ching trigrams and hexagrams
The trigrams and hexagrams are the building blocks of the I Ching. They're simply stacks of lines: a trigram is a stack of three lines -
- while a hexagram is a stack of six:
There are two kinds of line: broken (
), and solid (
). So, with all the possible arrangements of these lines, there can only be eight trigrams, and sixty-four hexagrams.
Follow the links for commentary on individual trigrams and hexagrams.
The Chinese word for both 'trigram' and 'hexagram' is gua. It means a 'figure', a divinatory symbol. The ancient Chinese character for 'gua', 
, includes the symbol for 'divination', 
, representing the cracks made in bone or turtle shell in the old way of divining. Its other part, gui, is widely said to be merely phonetic, without meaning of its own. LiSe Heyboer has more to say about this in her fascinating article, 'From Gui to Gua'.
The I Ching came into being when the hexagrams and texts were brought together: the combination of structures and imagery create meanings. It's not known which came first, figures or words, though this - along with which is the true key to interpretation - is endlessly debated.