In Eight Cyclical Character divination, the Heavenly Stems and Earthly Branches are paired in various permutations to record the year, month, day and hour of a person's birth. The most important pair is that which defines the day of birth; of the four columns, this is the fundamental one while the other three are for additional reference.
八字算生辰，不管年、月、日、時，都分別用天干、地支各取一字來記錄，其中又以「生日」最重要，稱為「本命柱」，其他的年、月、時三柱則供參考。