Upper trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
Hexagram 55 of 64
☳☲ Abundance
Fullness at its peak, which cannot last.
Lower trigram
☲
Fire · Li
Clarity and attachment.
FamilyMiddle daughter
ElementFire
The image
Thunder above, Fire below.
Thunder over Fire: arousal and shock meeting clarity and attachment.
That combination is the whole meaning of hexagram 55. 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.