Vertical / Horizontal

What is happening

The horizontal line being compared runs the full width, not just half of it. The vertical line runs from the intersection up to its top end. These two distances are made equal. Looking at the square that appears when you reveal the answer confirms that the vertical and horizontal lines fit into equal side lengths.

Why it looks that way

Length estimates aren’t always the same in vertical and horizontal directions. In this figure, the horizontal line also appears divided at the center. Because the effect of orientation and the effect of being split overlap, the difference you perceive can’t be pinned on a single cause.

How to check

Hold your phone screen a little farther away so you can see the whole figure at once when comparing. You can also try comparing the impression before and after measuring the lines with your finger, or after seeing the guide. This is not the kind of test that reveals anything about personality or ability.

See another geometric illusion

First described

After Adolf Fick, 1851
Source: Doctoral thesis, University of Marburg (published 1852)

Showed that a square appears taller than it is wide, and argued this could not be explained by the eye's optics alone.

See it in the timeline