Derived from a "common" inversion illusion, these paintings reflect the complex composition offered by a section of a cityscape where neither the sky nor the ground, are made available for the viewer. The work of Armand Thiery on the perception of rhombuses (which has been used in countless variations by artists since his work was published in 1895 - and unknowlingly before then) was particularly important to the development of this work. Thiery's tests conclude that this shape appears to people as a highly ambiguous figure, in which two cubes continuously and alternately assert either their concave or convex form.. This geometry was an appropriate vehicle for depicting the density of Manhattan at first, and Cairo in later paintings. The accidental title is taken from cross-sections of urban rooftops where buildings for worship can be seen.