Waterfall, 1961

Escher's print illustrates the "impossible triangle" described by the British mathematician Roger Penrose in a 1958 article on visual illusion: "Here is a perspective drawing, each part of which is accepted as representing a three-dimensional, rectangular structure. The lines of the drawing are, however, connected in such a manner as to reproduce an impossibility. As the eye pursues the lines of the figure, sudden changes in the interpretation of distance of the object from the observer are necessary."