If “breathtaking” were a building, it could very well be the Sainte-Chapelle (“Holy Chapel”) in Paris, the spectacular royal chapel adjoined to King Louis IX’s palace. The chapel was commissioned between 1242 and 1248 to house the Passion relics, including Christ’s Crown of Thorns, which King Louis IX purchased in 1239. The chapel is a perfect example of Rayonnant Gothic (circa 1240–1350) architecture. It was a style that aimed for structural lightness, where windows almost replaced the walls and flooded the buildings with light, and which favored a repetition of decorative motifs in varying sizes. Sainte-Chapelle is dominated by huge lancet windows that wrap around the upper chapel and reach up to heaven—well, up to 17 yards, so part of the way. Over the west door of the chapel is an exquisite 15th-century rose window in the Flamboyant style, a florid design of the late Gothic period in France. The window is framed by tracery, the intricate stonework that supports the …