Look at this image of a door to which a part has been hidden.
Do you think both sides coincide at the top?
Although it may not seem so if we uncover the door completely we see that both sides fit perfectly.
Poggendorff's illusion is based on the optical effect that occurs when an inclined line is interrupted in a segment of a certain length that produces the effect that the resulting pieces do not belong to the same line.
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