Upper trigram
☱
Lake · Dui
Joy and openness.
FamilyYoungest daughter
ElementMetal
Hexagram 17 of 64
☱☳ Following
Adapting to circumstances rather than forcing them.
Lower trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
The image
Lake above, Thunder below.
Lake over Thunder: joy and openness meeting arousal and shock.
That combination is the whole meaning of hexagram 17. 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.