"Meanwhile I have started on a new print; I got the idea for it from one of the learned professors of mathematics with whom I got on so well, as frère et compagnon, during my Amsterdam exhibition. The title will probably be Convex and Concave, and it is concerned with the widely known phenomenon of spatial suggestion which can be imagined as convex or concave, as desired. In the middle of the picture I draw the shapes in such as way that the observer may just as well see them convex as concave; to the right I force him to see things in a convex way (for example, 'cube from the outside'); to the left, he has to view things in a concave way ('cube from the inside')."

-M.C. Escher, December 1954.