Yijing Apprentice 3. Trigram Family. Poem by Makoto Ogino

Qian(乾) is father, bottom-middle-upper, three Yang lines,
Zhen(震) is eldest brother, two Yins on a bottom Yang line,
Xun(巽) is eldest sister, two Yangs on a bottom Yin line,
Kun(坤) is mother, bottom-middle-upper, three Yin lines,

Kan(坎) is middle brother, a middle Yang between two Yin lines.
Gen(艮) is youngest brother, two Yins under a top Yang line.
Dui(兌) is youngest sister, two Yangs under a top Yin line.
Li(離) is middle sister, a middle Yin between two Yang lines.

Yin and Yang usually behave contrasting.
But they also work, together in harmony.
Trigrams have this principle, inside of them.

You should treat them carefully when Hexagram casting.
Two Trigrams make a Hexagram like weaving symphony.
And remember, Hexagrams have Trigrams, inside of them.