Upper trigram
☷
Earth · Kun
Receptive yielding.
FamilyMother
ElementEarth
Hexagram 15 of 64
☷☶ Modesty
The mountain beneath the earth; strength kept low.
Lower trigram
☶
Mountain · Gen
Stillness and limit.
FamilyYoungest son
ElementEarth
The image
Earth above, Mountain below.
Earth over Mountain: receptive yielding meeting stillness and limit.
That combination is the whole meaning of hexagram 15. 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.