The Five Elements appear not as five but as ten — each element in both its yang (active, outward, expansive) and yin (receptive, inward, contracting) form. These ten Heavenly Stems (天干, tiān gān) are the most fundamental characters in BaZi: they appear in every pillar, they define the Day Master, and they carry the personality qualities that shape how elemental energy is expressed. Understanding them is the beginning of understanding BaZi.
Your Day Master is the Heavenly Stem of your birth day — the single most important character in your BaZi chart, the lens through which everything else is read. Enter your birth date to find it. Note: if born between 23:00 and midnight, the BaZi day may shift to the following calendar day (the Rat Hour 子時 begins at 23:00).
In BaZi, the Day Master is the reference point from which all other characters are read. Every other character in the chart is understood in relation to it — as something that produces it, controls it, supports it, drains it, or relates to it in one of the ten recognised ways (the Ten Gods, 十神). The same character means something entirely different depending on which Day Master it sits beside.
This is the core reason why BaZi cannot be reduced to "you are a Yang Wood person." Your Day Master tells you your elemental identity; the rest of the chart tells you what that identity encounters, is strengthened by, is challenged by, and becomes through the circumstances of your specific birth moment and the Luck Pillars of your specific life. The Day Master is the subject of a story; the rest of the chart is the story itself.
What the Day Master reveals at the most basic level: your core motivational structure, your natural approach to challenge, your instinctive relationship to the five elements, and the elemental "medicine" your chart is most likely to benefit from. A weak Day Master needs support from its productive element (Water nourishes Wood). A strong Day Master needs the outlet of expression (Wood produces Fire) or the governance of its controlling element (Metal governs Wood). Strength or weakness of the Day Master is determined by the Month Branch — and this is already one level beyond what a Day Master identification alone can tell you.
The Day Master calculator above gives you the starting point — not the analysis. Knowing you are 甲 Yang Wood tells you your elemental nature and suggests your core archetype. It does not tell you whether your Yang Wood is strong or weak, which elements support or oppose you, what your Luck Pillars activate, or how your Day Master interacts with your partner's chart. For the complete picture, you need all four pillars — and the understanding to read them together. The Ten Heavenly Stems section of this page prepares you to recognise your character; the rest of the BaZi section teaches you how to read the chart it anchors.