Most people see the vertical line as longer than the horizontal one even when it is shorter. Set the slider until they look equal, then check.
Try this
- Move the slider until the two lines look equal to you.
- Then tick Show the answer. Most people stop at about 85 to 90 percent: they make the vertical line shorter because it looks longer.
Why it happens
Part of the effect comes from the vertical line cutting the horizontal one in half (the "bisection" effect), which makes the cut line look shorter. But even an L shape shows a bias: vertical extents simply look longer. Explanations include the shape of our field of view, which is wider than it is tall, so a vertical line fills more of it, and the way we judge height in the real world. Gail Avery and Ross Day tested how much each part contributes.
Did you know?
Adolf Fick described the effect in 1851. It is also why tall, narrow pictures and stripes can make a space look taller.
References
- Avery G., Day R. (1969). Basis of the horizontal-vertical illusion.. Journal of Experimental Psychology, 81, 376-380. doi:10.1037/h0027737
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Why do vertical lines look longer?
Our visual field is wider than it is tall, and a line that cuts another in half looks shorter. Both make the vertical look longer.
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