The café wall illusion makes perfectly parallel rows of bricks look tilted into wedges. The recipe is exact: alternating dark and light bricks, rows staggered by about half a brick, and mortar lines in an intermediate gray between the two brick tones. Psychologist Hugo Münsterberg recorded a version in 1894, and in 1898 A.H. Pierce described it as the kindergarten illusion after the woven paper mats children made in class. The modern name came in 1973, when Steve Simpson from Richard Gregory’s lab noticed the tiles on a café at the bottom of St. Michael’s Hill in Bristol.

For years the tilt was blamed on irradiation, where bright areas appear larger than dark ones as light scatters in the eye. When researchers swapped black and white for red and green bricks of equal luminance, the illusion vanished, pointing to brightness. Yet part of the effect survives even after optical scattering is accounted for, and the modern explanation centers on contrast polarities: the visual cortex averages tiny local edges along the mortar and concludes the whole line slopes.

• With pure black or pure white mortar the illusion does not appear, because the lines give the brain clear boundaries.
• A normal checkerboard does not tilt, because irradiation pushes evenly in every direction.
• Staggering the rows makes the apparent expansion of white bricks press unevenly along the mortar line.
• The illusion depends on peripheral vision, where the brain blurs edges and averages shadows.
• A building in the Melbourne Docklands uses the pattern on its facade so its level lines appear to sag.