Design AtlasOverlap two dense, regular patterns — halftone dots or stripes — at even a slightly mismatched angle or spacing, and the two grids reinforce and cancel each other in a way that produces a coarse rippling pattern neither grid actually contains. It's optical interference, not a printing flaw in either layer alone.
In print it shows up most often when an already-halftoned image (from a newspaper or magazine page) gets rescanned and reprinted — the scanner's dot angle clashes with the original's. That's exactly why the four CMYK plates in process printing are each set at a different angle (commonly around 15°, 45°, 75°, and near 0°) — spacing the angles out is what keeps the dots from moiré-ing against each other.
The same thing happens photographing finely striped fabric, or a screen with its own pixel grid. The demo overlaps two identical stripe layers and slowly rotates one — watch the ripple pattern swell and fade as the angle drifts.
When to use
Watch for it when rescanning an already-printed image, or photographing striped or checked patterns. Nudging the angle or slightly lowering resolution both help suppress it.