Sander illusion

The Sander illusion or Sander's parallelogram is an optical illusion described by the German psychologist Friedrich Sander (1889-1971) in 1926. However, it had been published earlier by Matthew Luckiesh in his 1922 book Visual Illusions: Their Causes, Characteristics and Applications.

The diagonal line bisecting the larger, left-hand parallelogram appears to be considerably longer than the diagonal line bisecting the smaller, right-hand parallelogram, but is in fact the same length.