The eight characters are composed of heavenly stems and earthly branches in the year, month, day and hour. Ten days dry with twelve branches.
Year, month, day and hour are composed of eight words, so they are called birthdays, usually based on day and hour.
The first word is heavenly dryness, such as A, B, C and D, and the last word is earthly support, such as ugly yin.
For example, someone: Kun Zao (female) Ding Jiazi Day Bing Yin Day Geng Wu for four years.
Where d, a, c and g are the heavenly stems of a certain day in a month,
Fourth, children, yin and noon are their earthly branches.