Each slanted middle slides straight out to the right: they line up exactly, all the same slant.
Straight, bent or tilted?
Lipps illusion
Are the thick lines parallel?
Yes: the slanted middle of every zigzag is at exactly the same slant. They look as if they spread apart.
Why? The bends at each end make each middle look tilted, the way the short lines tilt the long ones in the Zöllner illusion. Theodor Lipps studied it in the 1890s; this is our drawing of the picture in his Wikipedia article.
Picture: Drawn by Science Playground, after Theodor Lipps · Wikipedia(opens in a new tab)