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Size and shape

Café wall illusion

Rows of black and white tiles make perfectly straight, parallel lines look like wedges.

By Illusions Blog · Updated · How we make them

TypeSize and shape
InteractiveYes: sliders and a check
First described1979 (studied)

The gray lines between the rows are straight and parallel, but they seem to slope.

Try this

  • Slide Shift each row to 0: the slant disappears. Half a tile is the strongest.
  • Change Mortar line brightness: it is strongest when the line is between black and white, and fades when the line is pure black or pure white.
  • Tick Show straight guides.

Why it happens

Richard Gregory and Priscilla Heard studied the effect on the tiled wall of a café in Bristol, England. The key is the thin gray "mortar" line: where it meets a dark tile on one side and a light tile on the other, small-scale processing in the eye and brain blurs the edges together into tiny wedges. Lined up along the row, the wedges add up to a tilt. Make the mortar line pure black or pure white and the illusion collapses, which is how they showed the line's brightness is essential.

Did you know?

The original café wall was at the bottom of St Michael's Hill in Bristol. A similar pattern was already known as the "shifted chessboard" in the 1800s.

References

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

Frequently asked questions

Are the lines in the café wall illusion straight?

Yes, and parallel. Turn on the guides to see.

Why does the gray line matter?

The illusion needs a mortar line between the tile shades; pure black or white lines remove it.

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