A Penrose stairway is an impossible figure named after by the British geneticist Lionel Penrose (1898-1972), father of Roger Penrose. It served as an inspiration for the staircase in M. C. Escher's famous print "Ascending and Descending." Although the Penrose stairway cannot be realized in three dimensions, this impossibility is not immediately perceived and, in fact, the paradox is not even apparent to many people at a quick glance. Although Escher and the Penroses made the Stairway famous, it was, unbeknownst to them, independently discovered and refined years before by the Swedish artist Oscar Reutersvärd. In the 1960s the Stanford psychologist Roger Shepard created an auditory analogue of the Stairway.