TypeSize and shape
InteractiveYes: sliders and a check
First described1860
The two parts of the blue line are perfectly lined up, but the right part looks too high or too low.
Try this
- Make the bar wider with Width of the bar: the misalignment grows.
- Tick Make the bar see-through to see the line runs straight.
Why it happens
The brain misjudges the sharp angle where the line meets the edge of the bar, and has to "fill in" the line's path across the hidden gap. Richard Day and Robert Dickinson broke the effect into parts and found that both the angle misjudgment and the gap contribute. It works best when the line is shallow and the bar is wide.
Did you know?
Johann Poggendorff, editor of the physics journal where Zöllner published, spotted this effect in Zöllner's figure in 1860.
References
- DAY R., DICKINSON R. (1976). THE COMPONENTS OF THE POGGENDORFF ILLUSION. British Journal of Psychology, 67, 537-552. doi:10.1111/j.2044-8295.1976.tb01545.x
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Is the line in the Poggendorff illusion straight?
Yes. Make the bar see-through to check.
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