There are four pillars in the Ba Zi chart. Each pillar has an animal sign branch at the bottom and one of the five element stems (in either yin or yang expression) on the top.
The first pillar is calculated on the year of birth, so it is called the year pillar.
The second pillar is calculated on the month of birth, so it is called the month pillar.
The third pillar is calculated on the day of birth, so it is called the day pillar (or "daymaster").
The fourth pillar is calculated on the hour of birth, so it is called the hour pillar. (It is important to adjust the time of birth to true local time-for example, if daylight saving time was in effect, this must be taken into account.)
The year pillar is calculated using the Chinese solar year calendar (rather than the Chinese lunar calendar, which begins anywhere from mid-January to mid-February on a different date each year), which is based on the return of the sun and the subsequent cycles of the sun's orbit.
The Chinese solar year begins on February 4 or 5, so if you were born in January in 1959, you would do your year pillar calculations based on 1958, an Earth Dog year.
The four pillars can show family background; relationship with parents, grandparents, children; marriage and romances; and relationships with bosses, peers, partners, and subordinates.
In each pillar, life qi combines, clashes, strengthens, weakens, assists, and controls in a complex tango of energies.