The four pillars of Ba Zi are Year, Month, Day, and Hour. Each pillar contains a Heavenly Stem and an Earthly Branch, producing the eight characters (八字) that encode a person’s birth chart.
Short answer
The four pillars of Ba Zi are Year, Month, Day, and Hour. Each pillar contains a Heavenly Stem and an Earthly Branch, producing the eight characters (八字) that encode a person’s birth chart.
In depth
The Year Pillar represents your ancestors and early life context (roughly ages 0–16). It places you within the 60-year sexagenary cycle and reveals your inherited karma.
The Month Pillar represents your parents and the season of your birth (roughly ages 17–32). It is often the most influential pillar because the month determines your chart’s elemental balance through the Solar Term system.
The Day Pillar represents you and your spouse (roughly ages 33–48). The Day Stem is your "Day Master" — the elemental representation of your core self, used as the reference point for all chart interpretation.
The Hour Pillar represents your children and later life (roughly ages 49+). It is the most personal of the four because it requires accurate birth time to compute correctly.