Two horizontal-striped dresses use different gaps. Why can their width illusion differ?

Answer: Line-gap ratio changes it

Line-gap ratio changes it ✓ — Right. Same direction does not mean same visual pattern: line width, gap width, and their ratio change how the eye groups the stripes. The Helmholtz literature treats width/spacing ratio as part of the illusion, and a 2026 dress study tested multiple spacing types rather than treating all stripes alike. That is the useful surprise. A stripe is not just horizontal or vertical; it is a small geometry system.

Direction alone decides — No. Direction matters, but direction alone is too crude. If two dresses are both horizontal yet use different gaps, the viewer is seeing different repeated geometry. The cited work explicitly treats width, spacing, and spacing type as variables. So horizontal-versus-vertical is only the first layer of the illusion, not the whole mechanism.

More white gap slims — No. More white gap may help in one design, but 'more gap always slims' is too simple. The evidence points to ratio and configuration, not a one-variable rule. Very wide spacing can make the pattern read differently from close pencil stripes. That is why the correct answer is the line-gap ratio, not just bigger gaps.