The impossible cube or irrational cube is an impossible object. It is represented as a 2-dimensional drawing of a cube, seen in an isometric perspective (parallel edges in the cube are parallel lines in the drawing).
The cube is drawn as solid beams, which cross in inconsistent ways, contradicting each other. The result is an object with 12 edges.
Necker cube on the left, impossible cube on the right.
In M. C. Escher's lithograph Belvedere, the figure of a boy seated at the foot of the building is holding an impossible cube.
The Necker cube is an ambiguous picture that consists of a wire-frame drawing of a cube in isometric perspective. It is interesting because people usually do not see an impossible cube when looking at it. People usually see one of two ordinary cubes.
See also: the Penrose triangle.