Penrose stairs (or Penrose steps) is an impossible object created by Oscar Reutersvärd in 1937. Later, independently discovered and made popular by Lionel Penrose and his son Roger Penrose.
The "continuous staircase" was first presented in an article that the Penroses wrote in 1959, based on the Penrose Triangle published by Roger Penrose in the British Journal of Psychology in 1958.
M. C. Escher 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.