Café wall illusion

The café wall illusion is an optical illusion, first described by Richard Gregory. When offset dark and light tiles are alternated, they can create the illusion of tapering horizontal lines. The effect depends on the presence of a visible line of gray mortar between the tiles. It is known as the "café wall illusion" since being observed on the outside wall of a café.

This illusion is created when offset rows of alternating dark and light tiles are surrounded by a visible line of mortar. Ideally, the mortar is a shade somewhere between the two tile colors. When the tiles are offset by half a tile width, the horizontal lines appear to slant diagonally, creating the appearance of wedges. The illusionary effect is affected by both the position of the tiles as well as the thickness and color of the grout in between them. If grout lines are removed, there is no longer any illusion of diagonal lines. (New World Encyclopedia)

Café wall illusion: The horizontal lines are parallel & straight, even if they seem otherwise.