Upper trigram
☶
Mountain · Gen
Stillness and limit.
FamilyYoungest son
ElementEarth
Hexagram 27 of 64
☶☳ Nourishment
What you take in, and what you say.
Lower trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
The image
Mountain above, Thunder below.
Mountain over Thunder: stillness and limit meeting arousal and shock.
That combination is the whole meaning of hexagram 27. Nothing about it is arbitrary — change either trigram and you get a different hexagram with a different reading, which is why the 64 form a system rather than a list.