In the above figure, the sides 
and 
have the same length, appearances to the contrary.
In the related illusion illustrated above, the interior lines appear to be of different lengths, despite the fact that they are the same (Wells 1991).
Parallelogram Illusion
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References
Wells, D. The Penguin Dictionary of Curious and Interesting Geometry. London: Penguin, pp. 86-87, 1991.Referenced on Wolfram|Alpha
Parallelogram IllusionCite this as:
Weisstein, Eric W. "Parallelogram Illusion." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ParallelogramIllusion.html