TypeSize and shape
InteractiveYes: sliders and a check
First described1891
The blue and yellow shapes are exactly the same size and shape. Whichever is on the bottom looks larger.
Try this
- Tick Swap their places: now the other one looks bigger.
- Tick Line them up to set them side by side.
Why it happens
Your eyes compare the short inner curve of the top shape with the long outer curve of the bottom shape, right where they sit next to each other. That nearby comparison wins over the overall size. Psychologist Joseph Jastrow described it in 1891.
Did you know?
The same shape shows up in toy train track pieces, which is how many kids discover it.
References
- Jastrow J. (1891). Studies from the Laboratory of Experimental Psychology of the University of Wisconsin. The American Journal of Psychology, 4, 198. doi:10.2307/1411267
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Are the Jastrow shapes really the same size?
Yes. Swap them and the other one looks bigger.
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