An isometric illusion (also called an ambiguous figure or inside/outside illusion) is a type of optical illusion, specifically of multistable perception. In the example figure at right, the shape can be perceived as either an inside or an outside corner. In general, any shape built entirely of same-length (i.e., isometric) lines that does not clearly indicate relative direction between its components will evoke such a perceptual "flip-flopping".

The Necker cube is a famous example of an isometric illusion.

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