I Ching - Yin is first?

Q: Why does Yin come first in The Nu I Ching, and Yang last? - F.P.

A: You will find a detailed answer in the book, where Yin is 0 (zero) and Yang is 63.  Briefly, Yin ("the receptive") is empty like space is empty, like the ground before it is seeded.  Yang ("the creative") needs this space in which to create, and cannot create without it.  Yin sets the stage for creation, and Yang is the culmination, the completion of creation.