The Four Pillars are written in the sexagenary cycle, the ganzhi, formed by pairing the ten Heavenly Stems with the twelve Earthly Branches. The stems and branches turn together like two meshed wheels, and because a yang stem can only meet a yang branch and a yin stem only a yin branch, the pairing yields sixty combinations rather than a hundred and twenty. This run of sixty, the jiazi, names years, months, days and hours alike.
A birth produces four such pairs, the four pillars:
- the year pillar, set by the solar year, which turns at the Start of Spring in early February, not at lunar new year;
- the month pillar, set by the solar term the birth falls in;
- the day pillar, counted continuously through the sexagenary day-count;
- the hour pillar, set by the double-hour of birth.
The stem of the day pillar is the day master, the character that stands for the person. Everything else in the chart is read as it relates to this one stem: what strengthens it, what drains it, what it controls and what controls it. Each of the sixty pairs also carries a nayin, a five-agent melody named in poetic images such as metal-in-the-sea or fire-in-the-hearth, a further layer of element assignment used in some methods of the reading.