geng-zi day pillar

Metal gives rise to water in the generating cycle, and 庚子 rests yang metal on 子, the Rat, a branch this site's table treats as water alone. Its single hidden stem is 癸, which to 庚 is the Hurting Officer: yin water the metal produces, at opposite polarity. The name sounds rough and records only that relation. 三命通會 would give 壬 a share of seven days before 癸 takes over.

三命通會 卷一 says of this pair 土中空者，屋宇也: earth that is hollow inside, which is to say a house. That fits the 納音 Earth on the Wall, a phrase it shares with 辛丑, and the entry adds that this earth likes wood and metal.

This site's count places 庚 at Death, 死, on 子, and the term simply follows Sickness in the sequence of twelve. The branch that marks it for 庚 is the one where 辛 starts its Growth, the same mirrored pairing of yang and yin found across the table.