If Héng has appeared in your reading, the book is naming the quality that allows something to last. The character means duration, constancy, the long term. The hexagram comes immediately after Xián (Influence) in the King Wen sequence, and the pair forms a unit: first the meeting that joins two things, then the constancy that lets the joining hold over time.

Classical commentary reads this hexagram as the work of the marriage proper, as distinct from the work of the courtship. The honeymoon is brief; the durable form is what comes after. Thunder above, wind below — both in motion, both keeping faith with the direction they were going. The hexagram appears when the question is not about beginning but about continuing.

What the book counsels is the discipline of not changing course out of restlessness. 立不易方 — stand firm and do not change direction. This is not rigidity. The thunder and wind are moving the whole time; what is constant is the orientation, not the absence of motion. The reader is being asked to sustain a direction long enough for the direction itself to bear fruit.

Héng's failure mode is the small reorientation undertaken to relieve the feeling of repetition. The book treats this as one of the chief enemies of any long undertaking. The hexagram appears when the reader is somewhere in the middle of something — past the excitement of the start, well before the satisfaction of the arrival — and the question is whether the going-on will continue. The clearance is unconditional. Keep going.