An Impossible Figure (also called the Schroeder Stairs) in which a stairway in the shape of a square appears to circulate indefinitely while still possessing normal steps. The Dutch artist M. C. Escher included Penrose stairways in many of his mind-bending illustrations.
See also Impossible Figure
References
Hofstadter, D. R. Gödel, Escher, Bach: An Eternal Golden Braid. New York: Vintage Books, p. 15, 1989.
Jablan, S. ``Impossible Figures.'' http://members.tripod.com/~modularity/impos.htm.
Pappas, T. ``Optical Illusions and Computer Graphics.'' The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra,
p. 5, 1989.
Robinson, J. O. and Wilson, J. A. ``The Impossible Colonnade and Other Variations of a Well-Known Figure.''
Brit. J. Psych. 64, 363-365, 1973.