A gauge that fits between two long lines slides all the way along: they’re parallel.
Straight, bent or tilted?
Zöllner illusion
Are the long black lines parallel?
Yes, all of them, though they seem to lean toward and away from each other.
Why? The short crossing lines make your brain exaggerate the angles where they meet, and that twists the long lines.
Picture: Fibonacci, CC BY-SA 3.0 · Wikipedia(opens in a new tab)