In the blown-up sketch above, note how the square on the left is distorted, while the square on the right appears normal. The specific problem with the square on the left is that its lower left vertex is too sharp, less than 90 degrees. However, the superimposed chevron pattern distorts our judgement. The square on the left is normal, while it is the one on the right that is distorted. This is properly called the Kmunicek Illusion. By the way, do you think this illusion is a variant of the better-known Zöllner illusion? If so, it works better! For a nice page with more than a dozen animated gifs of standard optical illusions, including the one shown here, click this.