The Café Wall Illusion, also known as the Münsterberg illusion, is a visual phenomenon where straight, parallel lines appear to be slanted due to the arrangement of alternating black and white tiles in staggered rows. This illusion is accentuated by a thin line of gray “mortar” separating the rows.
The effect was first described by Hugo Münsterberg in 1894 and later rediscovered by Richard Gregory in 1973. Gregory noted this illusion on a tiled wall at a café on St. Michael’s Hill in Bristol, England. The illusion’s mechanism is not fully understood, but it involves interactions in the visual cortex, where the brain processes the orientation of lines.