Poggendorff Line

What is happening

First, try mentally extending the line from the left side behind the band, and predict where it should emerge on the right. Does the actual right-side line look higher or lower than your prediction? This figure is drawn as one single line first, with a gray rectangle overlaid on top of it afterward — the two halves are not two separate lines placed with an offset.

Why it looks that way

When a diagonal line is interrupted at the edge of a band, your visual system has to estimate how the hidden part connects. The angle of intersection and the shape of the occluding band influence that direction judgment, so a truly straight line can end up feeling offset. It’s a difference between measuring a line and filling one in.

How to check

Removing the context reveals the hidden segment. Revealing the answer also adds a dotted line along the same path, confirming it runs straight from end to end. Try bringing the band back and see whether the earlier impression of misalignment also returns.

See another geometric illusion

First described

After Johann Christian Poggendorff, 1860

Poggendorff, the journal editor who received Zöllner's letter about his own illusion figure, noticed this separate illusion in it.

See it in the timeline