Zöllner Lines

What is happening

Trace one long line at a time from top to bottom. Then step back and compare the columns of lines against each other. Each one looks straight on its own, yet the whole set can look tilted. Every long line has the same horizontal position at top and bottom, and the spacing between neighbours is constant.

Why it looks that way

Judging a long line’s direction is also influenced by the angle of the short lines crossing it. Flipping the tilt of the short ticks in alternating columns can make parallel long lines feel like they’re drifting apart or closer together. It’s a phenomenon that shows how direction-reading interacts with context.

How to check

Hiding the short diagonal ticks leaves only parallel columns. Revealing the answer highlights the long lines in color. At a display size small enough that the fine diagonals get crushed together, the effect is hard to judge, so try it at a distance where the whole figure is comfortably visible.

See another geometric illusion

First described

After Johann Karl Friedrich Zöllner, 1860

Noticed in a fabric pattern, and reported to the journal Poggendorff edited.

See it in the timeline