T-figure illusion

The blue line looks longer than the red line (left). The same lines next to each other (right)

The T-figure illusion or T-illusion is an optical illusion in which the vertical line appears longer than the horizontal one.

If the end of the vertical line meets the horizontal line, usually in the middle, the vertical line appears longer than the horizontal line. The illusion was named after the letter T , where the phenomenon occurs. It does not matter whether the horizontal line runs above, below or to the side of the other line.

The reason is that the apparently shorter line appears divided; the brain has difficulty measuring. The thicker the vertical bar, the shorter the horizontal line appears. This suggests that the brain considers the part of the horizontal line where the lines meet as part of the apparently longer line.

If both lines are the same color, the illusion comes out better.

Similar optical illusions are the Müller-Lyer illusion and the L illusion .