The ten Heavenly Stems
The Stems are Gap, Eul, Byeong, Jeong, Mu, Gi, Gyeong, Sin, Im, and Gye in Korean reading: 갑·을·병·정·무·기·경·신·임·계. They pair the Five Phases with yin or yang, creating two expressions of Wood, Fire, Earth, Metal, and Water.
A Stem is the visible upper character of each pillar. Readers often treat it as a clearer or more outward expression, while remembering that its meaning changes through every relationship around it.
The twelve Earthly Branches
The Branches are Ja, Chuk, In, Myo, Jin, Sa, O, Mi, Sin, Yu, Sul, and Hae: 자·축·인·묘·진·사·오·미·신·유·술·해. They correspond to years, months, days, and traditional two-hour periods and are also associated with the twelve zodiac animals.
Branches can contain hidden Stems, so they carry more than a single elemental label. This is one reason a real chart cannot be reduced to counting five colors.
Why the cycle has sixty combinations
Stems and Branches advance together in a repeating sexagenary cycle. Because there are ten Stems and twelve Branches, the compatible pairings return to the beginning after sixty steps.
In Saju, the pair assigned to each birth moment becomes one pillar. Calculating those pairs correctly requires calendrical rules, including solar terms and the traditional hour system.
Sources & further reading
Lucia’s guide is written in original language from the reference material below. Interpretive traditions vary by school and practitioner.