Another variant of the Wundt illusion is the Horizontal–Vertical Illusion, introduced by Wundt in 1858. The two intersecting lines are equal in length although the vertical line appears to be much longer. The horizontal line needs to be extended up to 30% to match the perceptual length of the vertical line. This is not confined to simple line drawings, as this can also be seen in buildings, parking meters, as well as other things viewed in a natural setting.
Another variant, Hering Illusion, produces a similar, but more inverted effect.