Upper trigram
☴
Wind · Xun
Gentle penetration.
FamilyEldest daughter
ElementWood
Hexagram 42 of 64
☴☳ Increase
A favourable time; act while it lasts.
Lower trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
The image
Wind above, Thunder below.
Wind over Thunder: gentle penetration meeting arousal and shock.
That combination is the whole meaning of hexagram 42. 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.