The Ten Heavenly Stems — Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, Gui — express the energy, mind and will that shows on the surface. Among them, the Stem of your birth day, the Day Master, is the single most important character, standing for "you" at the center of the chart.
Ten Stems, Yin-Yang and element
The ten Stems are the five elements each paired into Yang and Yin. Jia (甲) and Yi (乙) are Wood, Bing (丙) and Ding (丁) are Fire, Wu (戊) and Ji (己) are Earth, Geng (庚) and Xin (辛) are Metal, Ren (壬) and Gui (癸) are Water.
Even within the same element, the Yang Stem is large and outreaching (a great tree Jia, the sun Bing) while the Yin Stem is delicate and substantial (a flower Yi, a candle Ding). This Yin-Yang difference gives distinct textures to the same energy.
The Day Master — the protagonist
Casting a chart yields four pillars (year, month, day, hour), and the Stem of the day pillar is your Day Master. Every other character in the chart is interpreted by its relationship to this Day Master.
For example, someone with a Geng (Yang Metal) Day Master carries decisiveness and principle as their base, and how their life unfolds depends on how the other characters support and refine that metal energy.
Frequently asked
You can sketch the broad outline, but not the whole. The Day Master is your basic material; your actual nature is shaped together with the season of your birth month, the Ten Gods composition, and elemental balance. Treat it as a starting point.
Stems are surface energy (mind and will); Branches carry the underlying ground of reality and the flow of season and time. If Stems are "what you want to do," Branches are closer to "the ground you stand on."