A gua is an image composed of yin and yang. The flag of the Republic of Korea, shown to the right, is composed of a traditional circular yin/yang image surrounded by four gua of three yao (lines) each.
Lines (yao) that are solid represent yang while the broken lines are yin. The eight fundamental gua of three yao are known as ba gua (meaning ‘eight gua’), shown to the left. The 64 I Ching gua each have six yao, as shown below right, consisting of one upper three yao image and one lower three yao image from the ba gua (64 possible combinations of yin and yang exist in a six yao gua.)
The essential I Ching text commonly used today is attributed to King Wen of the Zhou Dynasty. The text, referred to as the Zhou I, consists of a brief verse for each gua and yao. The Zhou I text is usually published with additional commentary by sages such as Confucius. The most widely recognized of those commentaries are known as the Ten Wings.
Also attributed to King Wen is arrangement of the 64 gua of the I Ching in their most common presentation, referred to as the King Wen Sequence. Another sequence of the gua attributed by some scholars to Shao Yong, 1012-1077, is called Fu Xi’s arrangement.
Gottfried Leibniz (1646-1716, credited with refining the modern binary number system) was invited by a correspondent in China to examine Fu Xi’s arrangement. He saw that upon substituting a zero for each yin line and a one for each yang line (starting from the top down), Fu Xi’s arrangement represented a binary numbering sequence. The animated graphic to the right, created by I Ching scholar Steve Marshall, illustrates both the numerical sequence of the binary system and the sequence of the Fu Xi arrangement of the 64 gua.