Short answer
There are ten Day Masters (core elements), one per Heavenly Stem (sky sign): Jia and Yi (Wood), Bing and Ding (Fire), Wu and Ji (Earth), Geng and Xin (Metal), Ren and Gui (Water). Yours is the stem of the day you were born, and it is the core of your BaZi chart.
What a core element is
Your birth moment is written as 8 Chinese characters. The top character of your day pair is the one that stands for you. Classical books call it the Day Master; we call it your core element.
There are exactly 10, spread across the five elements. Each element has a Yang form (outward, bold, expansive) and a Yin form (inward, adaptive, refined).
It is not a zodiac sign or a quiz result. It comes from the day you were born, counted in a Chinese cycle of 60 days, so a normal calendar cannot show it. Together with the strongest force set by your birth month, it decides which of the 80 guardian spirits is yours.
For the curious: the proper names
- Day Master 日主
- The classical name for your core element.
- Heavenly stems 天干
- The 10 sky signs. Each core element is one of them.
- Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, Gui
- The Mandarin names of the 10 signs, from Tree to Rain.
- 60-day cycle 六十甲子
- The repeating count of days that sets which sign rules each day.