The eight characters are composed of heavenly stems and earthly branches, four accurate columns of year, month and day, and each column is composed of heavenly stems and earthly branches, so there are eight characters, so it is called eight characters.
Generally, heavenly stems and earthly branches has four words, Yin and Yang. If all eight words are yang, it is called full yang; If all eight words are yin, it is called full yin;