Horizontal/Vertical Illusion Description

The Horizontal/Vertical illusion demonstrates that a vertical segment appears longer than an equal-length horizontal segment when presented in a certain configuration.

Passing the mouse pointer over the following links will cause the length of the vertical segment of the figure at left to change.

( 130, 140, 150, 160, 170 )

Which setting appears to make the vertical line the same length as the horizontal? After you have decided, compare the segments by measuring them along a pencil or the edge of a piece of paper.

The effect also applies to the farthest points of (somewhat) more naturalistic stimuli, such as the tree with bushes and the hat shown below. (Compare the overall height to the largest horizontal width).

Research has shown that the apparent sizes of both parts of the stimulus are affected by their combined presentation. The vertical line seems longer than when presented in isolation, while the horizontal line seems shorter. The effect cannot be cognitively overridden; even if you realize there is an exaggeration, you cannot avoid the illusion.

The illusion is probably increased because the horizontal width is bisected by the vertical object, and thus may not be perceived as one unbroken whole.

[Note - The actual height/width ratio will vary, depending on the resolution of your monitor. We have chosen the lines on this page to cover a range of proportions. The other images may not be exactly square on your display.]