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Size and shape

Müller-Lyer illusion

Two lines of exactly the same length look different because of the fins at their ends.

By Illusions Blog · Updated · How we make them

TypeSize and shape
InteractiveYes: sliders and a check
First described1889

The line with the fins pointing outward looks clearly longer than the one with arrowheads. They are the same length.

Try this

  • Drag Fin angle toward 80°: the effect fades as the fins flatten.
  • Set Fin length to 0 and the lines suddenly match.
  • Switch on Show the truth to see guide lines at both ends.

Why it happens

No one explanation has won, but the leading ideas all say your brain is reading the fins as clues about depth or context. One idea is that the fins look like the corners of a room or a building: arrows pointing in suggest a near corner, fins pointing out suggest a far one, and your visual system scales up what it thinks is farther away. A related account, from Catherine Howe and Dale Purves, argues that in real-world images, lines with these kinds of endings usually do come from longer physical edges, so the brain reports the most likely length rather than the measured one.

The effect is not only about 3D corners, though: it still works with circles or squares at the ends, which is why researchers think several mechanisms add up.

Did you know?

Franz Carl Müller-Lyer published the figure in 1889. It is one of the most studied illusions in psychology, and researchers have found the strength of the effect varies between groups of people, which keeps the debate going.

References

The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.

Frequently asked questions

Why do the lines in the Müller-Lyer illusion look different lengths?

The fins act as context clues about depth and size; the brain adjusts the length it reports. The exact mechanism is still debated.

Does knowing the trick make it go away?

No. Even after measuring, most people still see one line as longer.

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