jia-zi day pillar

甲子 opens the cycle of sixty pairs: yang wood sitting on 子, the branch of water. Water feeds wood in the five-element cycle, so the seat supports the stem above it. The method places 甲 at Bath in 子, and 五行大義 already has wood there in its own list: 沐浴於子, it bathes at 子. 三命通會 explains the name through a newborn washed three days after birth, when the body is tender and easily hurt.

Hidden stems are the stems a branch is held to contain. The table used on this site gives 子 only 癸, which from 甲 counts as Direct Resource: water feeding wood at opposite polarity. 淵海子平 and 神峰通考 agree on that single stem, while 三命通會 lets 壬 rule the first seven days of the month and 癸 the remaining twenty-three.

The 納音, a separate element assigned to each pair of the sixty, is 海中金, Gold in the Sea, shared with 乙丑. 三命通會 卷一 writes 甲子金，为宝物, metal as a treasured thing, and adds 喜金木旺地: it is pleased by places where metal and wood flourish. That chapter speaks of the pair wherever it falls in a chart, and it lists the name 福星, Fortune Star, among several labels it leaves undefined.