Understanding the arch becomes a fascinating and delightful proposition under the guidance of Alain, who knows how to tell a story and to draw funny, exact pictures for it. From the legend of Monsicur Down, one of the French demons who for centuries brought any efforts at church building to no avail, comes the tale of a single young man who one day studying a particularly ruinous job of Monsicur Down's, discovered the idea of the Roman arch when he noticed how two big stones were firmly stuck together in a hole. There follows both a technically and historically careful account of their more complicated construction, and how the wish for more light and for a loftier church led eventually to the slim and pointed gothic arch. Monsieur Down makes his appearance at the end too, but only as an inactive gargoyle at the top of Notre Dame in Paris. A highly successful rendition of a complex subject on a simplified level.