Just a simple tilt illusion using angled lines… From the creator, greeenpro2009:
How this “works”: The sets of opposite diagonal lines in this drawing cause us to perceive the top rectangle as leaning toward the right when it really is not. Removing the lines proves that the rectangles are parallel. What also aids in the illusion is the triangle drawn on the right end of the top rectangle (we subconsciously perceive it as weighing down the right side…but only when the diagonal lines are there).
It’s fascinating the tricks that can be played on our brains.