What is BaZi? The four pillars explained

BaZi writes a birth moment as eight Chinese characters. Four of them describe the sky, four describe the earth, and together they are the chart a reading is built from. Here is what each character actually is, without the mystique.

The four pillars are four pairs

A birth moment is written as four vertical columns, one for the year, month, day and hour. Each column holds two characters: a heavenly stem on top and an earthly branch below. Four columns of two characters make eight, which is why the system is called BaZi — literally eight characters.

The pillars are not independent. The year frames the generation, the month frames the season, the day is the person, and the hour is the moment. A reading weighs them against each other rather than reading any one alone.

The ten heavenly stems

There are ten stems, alternating between yang and yin, and each is tied to one of the five elements. They are the yang and yin face of the same element, which is why there are ten rather than five.

The twelve earthly branches

The twelve branches are the familiar cycle of the zodiac, and each one also owns a two-hour window of the day. That window is what decides the hour pillar — which is why an error of an hour in the birth time can change the chart.

The window shown is the clock window. A calculator working in true solar time shifts these boundaries to match the sun at the birthplace.

The day master

The heavenly stem of the day pillar is the day master, and it represents the person the chart belongs to. A reading starts from it: which elements support it, which drain it, and whether the month it was born into strengthens or weakens it.

This is why the calculator shows the day master separately. If the birth time is wrong, the day master itself can be wrong, and every relationship in the reading shifts with it.

The five elements

Wood, fire, earth, metal and water are the vocabulary of change in Chinese thought. In a chart they are used relationally: wood feeds fire, fire makes earth, earth bears metal, metal carries water, water grows wood, and each element also controls another. A reading is largely a study of which of these movements are strong, weak or blocked.

Counting how many of each element appear is a starting point, not the answer. Position matters: an element in the month pillar, which fixes the season of the chart, carries more weight than the same element in the hour.

Why the calendar is the hard part

The rules above are simple. Applying them correctly is not, because BaZi does not use the civil calendar:

• The year changes at lichun, around 4 February, not on 1 January and not at Chinese New Year.
• The month changes at each jie solar term, so a month is roughly 30.4 days and the boundaries fall on dates that move year to year.
• The hour is decided by true solar time, not clock time, which brings in timezones, daylight saving, longitude and the equation of time.

Our true solar time page goes through the third point in detail, and the hour pillar page covers the boundaries and the midnight question.