Square and Diamond

What is happening

Glance quickly at the two squares — does the right one feel bigger? The box that encloses the right figure, measured vertically and horizontally, is indeed wider. But what we’re comparing here is the shape’s own area and edges. Consider separately what “bigger” is measuring.

Why it looks that way

When estimating a shape’s size, more than just its area gets folded into the judgment — how far it spans up-down and left-right matters too. Rotating a square increases the width and height of its bounding box, while the side length and area stay unchanged. That difference feeds into the impression of size.

How to check

Revealing the answer rotates the right shape back to its original orientation and overlays it on the left one in the same spot. Their outlines line up exactly, showing neither has been scaled. The important part of this experiment is not treating the difference in apparent width as an illusion too.

Try a related illusion

For reference: explanation and research. The figure here is an original implementation.

First described

After Ernst Mach, 1886
Source: "Beiträge zur Analyse der Empfindungen" (Contributions to the Analysis of Sensations)

The observation that the same shape reads as a different one when rotated 45 degrees.

See it in the timeline