It is supposed to give a theory of the phenomena of the physical universe, and of moral and political principles by the trigrams and the different lines and numbers of the hexagrams of Fuh-hi.
Or, in English, if a hexagram has complementary trigrams, then its correctness hexagram will have the same trigram repeated.
Of all the ancient books not one was more prized by him than the Yi-king, or " The Book of Changes," the rudiments of which are assigned to Fuh-hi about the 30th century B.C. Those rudiments, however, are merely the 8 trigrams and 64 hexagrams, composed of a whole and a broken line (, - -), without any text or explanation of them earlier than the rise of the Chow dynasty.
These are called trigrams and there are two in each hexagram.