Convex and Concave (March 1955) has a nightmarish quality: where is the entrance, are we going up or down, are we inside or outside? The construction in the middle, where two perspectives merge, evokes a sensation of dizziness.
Escher turns everything inside out: top becomes bottom, outside becomes inside. The people, lizards and pots he puts in this setting defy this inversion. Looking at them confronts the viewer with the impossible task of combining a convex and a concave world. Over and over again.