You have yin, you have yang, you have both representing the "one" or the "whole" of the two. Then you have the black side, the white side and then you will find in most renderings a circle surrounding the entire yin-yang, the symbol of the enso, i.e. the zen circle.
The enso surrounding the yin-yang symbol, as a part of the whole, is also representative of "infinity," and "the mirror" or the third jewel of Japanese culture and belief. It is also, by itself, indicative of the void.
So, see, the numerology is represented and representative of all things including the yin-yang. Now you have to ask yourself why is three important to the martial arts such as karate.
Heart, mind and spirit - the unification of the "three" to make a person a whole or wholehearted practitioner leading to other more as to wisdom, serenity and health.
|Click for larger view, look closely and "see" three.|