The ten stems
What are the Heavenly Stems?
The ten Heavenly Stems are the five elements split by polarity, one bold yang temper and one quiet yin temper each: the forest and the vine, the sun and the candle, the mountain and the garden, the axe and the jewel, the ocean and the dew. When the stem of your day pillar is one of the ten, it becomes your Day Master, the character that stands for you.
The yang stem is the big, wild version of its element; the yin stem is the small, tended version. Same element, two tempers. The picture is a teaching device for a temperament, drawn from the classical texts, not a literal claim.
Your stem is only a starting sketch. What turns it into a full reading is its strength and the relationships it forms with everything around it.