Café Wall

What is happening

Each row’s tilt runs opposite to the one above it, and the whole wall can look like it is buckling. Every figure here is built from horizontal rows of plain rectangles. Toggling the answer turns the mortar lines a bright color. Check that the endpoints sit at the same height on both sides, and that the spacing between rows never changes.

Why it looks that way

The black-and-white edges sitting next to the thin gray mortar line throw off your estimate of the line’s direction. The strength of the effect changes with the offset between rows, the width of the mortar line, and its brightness. It is not simply a diagonal line hidden underneath a straight one.

How to check

Removing the context clears the tiles away and leaves only horizontal lines. Flip back and forth between the two states to see how much the tile pattern was doing. If the mortar line gets too thin at a small display size, the effect weakens.

See another geometric illusion

First described

After Hugo Münsterberg, 1897
Source: „Die verschobene Schachbrettfigur“, Zeitschrift für Psychologie und Physiologie der Sinnesorgane

Richard Gregory and Priscilla Heard rediscovered it on the tiled wall of a café in Bristol and analysed it in "Border locking and the Café Wall illusion" (1979), which gave it its current name. It is also called the "kindergarten illusion".

See it in the timeline