The horizontal lines are parallel but appear to tilt; the acute angles formed by the horizontal lines and the short inducing lines appear to expand.
It is said that the illusion is maximized when the intersecting angle is between 10 and 30 degrees. Some references claim that any angle from 0 to 90 degrees produces this illusion, yet this was not supported by other references.
The Zöllner Illusion is characterized by acute-angle contraction, intensified by the oblique arrangement. It’s formed by three elemental illusions. Two are acute-angle expansion illusions; the third is an acute-angle contraction illusion.
Source: A. Kitaoka