Mental Floss

The Lines Are Perfectly Parallel in This Trippy Café Wall Illusion

Michele Debczak
Selco Builders Warehouse

If your neighborhood café had walls like the image above, you would question its structural integrity. The rows of "bricks" in the picture appear to be off-kilter, but if you take a closer look you'll see that they're perfectly parallel. This design from Selco Builders Warehouse is a classic example of the café wall illusion, which has been stumping observers for decades.

British psychologist Richard Gregory noticed this phenomenon in the real world in the 1970s. While visiting a café on St. Michael’s Hill in Bristol, he saw that the mortar lines in the tile wall seemed to be askew [PDF]. This was a consequence of how the tiles were arranged. Instead of making neat stacks, the pieces were offset by half a tile width, with the colors of each column alternating from dark to light. Though the horizontal mortar lines were parallel to the ground and to each other, this pattern made them look slanted.

Richard Gregory visits the original café wall in Bristol in 2010.
Richard Gregory visits the original café wall in Bristol in 2010. / StevenBattle, Wikimedia Commons // CC BY-SA 3.0

You don't have to travel to Bristol to experience the illusion for yourself. Selco has recreated the effect with a trippy illustration. According to a 2005 study, your faulty perception of the lines is likely a result of how the neurons in your brain's visual cortex interact to determine orientation. Experts aren't totally sure how the illusion works, which makes it even more mind-bending to look at.

The café wall illusion isn't the only deceptive imagery you can find in your everyday life. After teasing your brain with this picture, see if you can puzzle out this impossible-seeming photograph of a stack of chairs.