The "continuous staircase" was first presented in an article that the Penroses wrote in 1959, based on the so-called "triangle of Penrose" published by Roger Penrose in the "British Journal of Psychology" in 1958. M.C. Escher then discovered the Penrose stairs in the following year and made his now famous lithograph "Klimmen en dalen" ("Ascending and Descending") in March 1960. Penrose and Escher were informed of each other's work that same year. Escher developed the theme further in his print "Waterval" ("Waterfall"), which appeared in 1961.