The horizontal line segment in the above figure appears to be shorter than the vertical line segment, despite the fact that it has the same length.
Vertical-Horizontal Illusion
See also
Illusion, Müller-Lyer Illusion, Poggendorff Illusion, Ponzo's IllusionExplore with Wolfram|Alpha
References
Fineman, M. The Nature of Visual Illusion. New York: Dover, p. 153, 1996.Referenced on Wolfram|Alpha
Vertical-Horizontal IllusionCite this as:
Weisstein, Eric W. "Vertical-Horizontal Illusion." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Vertical-HorizontalIllusion.html