In the beginning, the entire universe is one mass.
It is called "Tai Ji" meaning "the Penultimate".
If one is to design a logo for Tai Ji, it would be a big circle like the above.
From Tai Ji, the universe is devolved into two states, or two "yi".
These two states (yi) are called "Yang" and "Yin".
Now we can design a more interesting logo for the "yi" of Yin and Yang.
People likes this so much better they adopt it as the logo for
Tai Ji as well.
As a matter of fact, this logo did not come out of the blue. It is
based on an ancient scientific recording of annual movement of the Sun, which looks
(1) Later, when people developed martial arts,
they decided to adopt the term "Tai Ji" as a brand name of
martial arts and calls it "Tai Ji Qian."
In truth, the connection between "Tai Ji" and "Tai Ji Qian"
is rather nebulous.