BaZi, also called the Four Pillars, represents the year, month, day and hour of birth with one Heavenly Stem and one Earthly Branch each—eight characters in total. A serious reading does not label each character lucky or unlucky in isolation. It studies the structure created by season, position and interaction.
What the four pillars represent
The Year Pillar records the year according to the Start of Spring boundary and often frames early family and social background. The Month Pillar changes at solar-term boundaries and is central to season and structure. The Day Stem is the Day Master—the reference point for the Ten Gods—while the Day Branch is one of the positions closest to the self. The Hour Pillar is set by true solar time and is often consulted for later development, results, children or inner plans, but none of these areas can be decided from a pillar label alone.
Traditional age associations for the four pillars are useful orientation, not a complete reading. The same role changes meaning according to position, seasonal support, roots, regulation and activation by luck cycles.
Why calendar months are not BaZi months
The Year Pillar changes at the Start of Spring rather than January 1 or Lunar New Year. BaZi months begin at twelve solar-term boundaries: the Yin month at Start of Spring, the Mao month at Awakening of Insects, and so on. One Gregorian month can therefore contain two different Month Pillars.
The Day Pillar also requires a stated rollover rule. This site changes the BaZi day at 23:00 true solar time and divides the Hour Pillar by the same solar clock. A method must remain fixed rather than being changed after seeing an inconvenient result.
What else a complete chart contains
The eight visible characters are only the surface. Branches contain hidden stems, and every visible or hidden stem receives a Ten Gods role relative to the Day Master. The reading then examines seasonal command, visible stems, roots, support, control, and combinations or clashes among stems and branches.
A count of the Five Phases records presence, not actual strength. One character with seasonal command, roots and support can matter more than several characters that are out of season and constrained. That is why a reliable report does not say “three Wood characters means Wood must be strongest.”