How can this illusion be 
explained? According to some 
investigators, the Poggendorff 
illusion, as well as many other 
illusion figures, is best 
explained by a third theory, 
one based on a type of contrast 
termed angular displacement. 
Investigators of illusions have 
long maintained that we tend 
to overestimate acute angles—–
that is, we tend to perceive 
them to be larger than they 
actually are. In the case of the 
Poggendorff illusion, 
overestimation of the two acute 
angles would cause the two 
oblique lines to appear to bend 
in opposite directions, as 
shown in the illustration at 
left. Thus, the oblique lines do 
not appear to be perfectly 
aligned.

Overestimation of the acute 
angles in the Poggendorff figure 
would lead to the illusory 
misalignment shown by the 
dashed lines.