Impossible Trident

The impossible trident, also known as the devil's tuning fork or blivet, is an ambiguous two-dimensional line drawing that appears to depict a solid three-dimensional object with two rectangular prongs at its base that impossibly become three cylindrical prongs at its top. It is a classic example of an impossible object: each local region of the drawing is a plausible interpretation of a real three-dimensional shape, but no single consistent three-dimensional object could produce the drawing as a whole, so the visual system cannot resolve the figure into one coherent shape no matter how long it is studied. It is used in psychology to illustrate how depth and shape perception work by locally consistent inference rather than by checking a scene's global consistency.