The letter is to count the year, month, day and hour of a family's birth, arrange them into eight characters in turn according to heavenly stems and earthly branches (called "eight characters"), and then calculate a person's fate with the five elements of the main branch. Decide people's good or bad luck and so on ~
Take a look at the academic works of fortune telling, including The Book of Changes, Geographical Discrimination, Divine Nature, Wei Zi's Doodle, and Addendum!