Commenting on what he described as `the function and meaning of backgrounds’ M. C. Escher noted: ‘It’s a remarkable characteristic of the recognizable figures of a periodic drawing…that our eyes never see a continuous pattern but fix their attention’. In this case, the black birds and white fishes ‘can never be seen simultaneously as objects, but function alternatively as one another’s background…The square is divided exactly into halves by a horizontal central strip in which the white and black components are equivalent…the white fish silhouettes merge to form the “sky” for the birds, while in the lower half the black birds blend together to form “water” for the fishes’. (The artist, in: Escher on Escher, Exploring the Infinite, Abrams, New York, 1989, pp. 35-36).