Three thousand years of notes on how situations change, one page each — traditional name, what the situation is, and what it asks of you.
Six lines, each solid or broken, give sixty-four states. Written as ones and zeros that is a six-digit binary number, which is what Leibniz recognised in 1701 when a missionary sent him the arrangement from China.
The relationships between the states are arithmetic, not interpretation. Reverse the six digits and you get how the same situation reads from the other end. Flip every digit and you get its opposite. Take the middle four and recombine them and you get what sits underneath. Do that last operation over and over and every one of the sixty-four collapses into the same four.
Even the order is built, not chosen: the sixty-four sit in thirty-two pairs, twenty-eight joined by reversal and four by inversion, with no exceptions. What the book is usually known for — casting for an answer — is the door. This is what is inside.