The blue diagonal in the larger part looks much longer than the one in the smaller part. They are the same length.
Try this
- Move Slant of the sides and watch both lines change length together.
- Tick Show only the two diagonals and compare them on their own.
Why it happens
Your brain judges the length of each diagonal partly by the size of the shape it sits in. The long diagonal belongs to a big parallelogram and the short-looking one to a narrow one, so the frame pulls your estimate up or down. It is related to the Müller-Lyer illusion, where the ends of a line change its apparent length. Westheimer's review of geometrical illusions puts it among the effects where the surroundings change judgments of length and position.
Did you know?
Friedrich Sander published the figure in 1926. Few length illusions are as easy to see.
References
- Westheimer G. (2008). Illusions in the spatial sense of the eye: Geometrical-optical illusions and the neural representation of space. Vision Research, 48, 2128-2142. doi:10.1016/j.visres.2008.05.016
- 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 two diagonals the same length?
Yes. Show only the diagonals to compare them.
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