A trigram is made up of three individual lines.
The eight possible three-line trigrams are the foundations of the I Ching - each of its six-line hexagrams being formed through the coexistence within it of two trigrams, making a total of 64 possible combinations.
The nature of a trigram's composition determines its character, as identified through its meanings, associations and state of change.
These are some of the more fundamental relationships, taken from Richard Wilhelm's introduction. Here I've linked each trigram to the hexagram in which it is found doubled...
Beyond these lies a world of further associations. They range from the archetypal examples above - which feature strongly in the I Ching itself - to the finest differentiations of everyday things, as favoured by astrologers and fortune tellers.