From straight to curved perspective
This is not to say that it was a simple task to make clever use of curved perspective. This first required an intermediate step: Escher produced two pencil studies in which he used straight perspective lines to create an impossible image. In these studies, he combines the vanishing point directly beneath you (the nadir, an extremely high angle) or the point directly above you (the zenith, an extremely low angle) with a straight-ahead view. They are, in fact, variations on Other World. In the centre, the perspectives intersect, rendering the image impossible. In a third study, he combines two different buildings in which the impossible point in the centre is a narrow street or (depending on your viewpoint) the sky. In this third study (in private collection), he introduces curved perspective lines. This makes the transition from a straight-ahead perspective to the nadir or the zenith much smoother, and your eyes begin to believe that this could exist in reality.
However, he realised that it made more sense to create a single building that could be experienced from two different viewpoints. The second stroke of genius was to have the central point fulfil two functions at once: ceiling and floor. In the final print, Up and Down, the nadir and the zenith converge in one and the same building, with both appearing twice. Thanks to the curved perspective lines, he repeats elements – such as windows, columns or a staircase – without becoming repetitive: the same element looks very different from the nadir than it does from the zenith.