An isometric illusion (also called an ambiguous figure or inside/outside illusion) is a type of optical illusion, specifically one due to multistable perception.
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.’Â The Necker Cube is a famous example of an isometric illusion.
The Daily Omnivore 
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