The nuclear hexagram (互卦, hùguà, "interlocking hexagram") is a hexagram hidden inside another one. It's built from the four middle lines, 2 through 5, which overlap:
- Lines 2, 3 and 4 become the new lower trigram.
- Lines 3, 4 and 5 become the new upper trigram.
The first and sixth lines are left out. That's why the nuclear hexagram is said to show the heart of the matter, what's going on inside and isn't visible at first glance.
An example
Take Initial Obstacles. From the bottom up, its lines are yang, yin, yin, yin, yang, yin. Lines 2 to 4 are three yin lines: Earth. Lines 3 to 5 are yin, yin, yang: Mountain. Mountain over Earth is The Fall. Underneath the hard beginning, something is crumbling, and it's worth letting it go.
Some curious facts
Only 16 hexagrams can be the nuclear of another. If you repeat the calculation again and again, you always end up at one of four: Activity (1), Passivity (2), After the End (63) or Transition (64). The first two are their own nuclear, and the last two turn into each other.
When to look at it
It isn't part of every reading. It's an extra tool for when you want to go deeper or the answer feels incomplete. There's more in The Nuclear Hexagram.