Several other well-known 
illusions, such as those shown 
to the left, illustrate this 
general principle. Even though 
the illusory effects appear to be 
different—–parallel lines appear 
to converge, straight lines 
appear curved, circles or 
squares appear distorted—–it has 
been held that they share a 
common denominator: A single 
test line appears to bend away 
from the inducing lines that 
cross it. The greater the number 
of crossing points, the more 
bent the line appears to be.