In 1239 King Louis IX of France bought the reputed Crown of Thorns from Baldwin II, the cash-strapped Latin emperor at Constantinople, who had pawned it to Venetian bankers. Louis paid 135,000 livres for the relic and another 100,000 for a silver reliquary chest, the Grande Châsse, while the Sainte-Chapelle built to hold them in his palace on the Île de la Cité cost 40,000. The purchase served politics as much as faith: with the Holy Roman Empire in disarray, Pope Innocent IV declared that Christ had symbolically crowned Louis, who cast himself as heir to Charlemagne. In August 1239 Louis carried the relics into Paris barefoot, dressed as a penitent.

The chapel is the peak of Rayonnant Gothic, with a 6.6 meter lower chapel for palace staff and a 20.6 meter upper chapel for the royal family, walled in 670 square meters of stained glass. Its 15 windows run from Old Testament kings to scenes of Louis himself acquiring the relics, and Louis was canonized in 1306, with part of his skull added as a relic. Revolutionaries later turned it into a grain store, smashed the fleurs-de-lis, and melted the Grande Châsse. From 1803 to 1837 it held court archives, and clerks had the bottom two meters of glass removed for light, which spurred the 1840 to 1868 restoration by Félix Duban, Jean-Baptiste Lassus, and Eugène Viollet-le-Duc.

• The hidden iron chains threaded through the stone, with lead-sealed joints, that hold the glass walls together without flying buttresses
• Louis Grodecki’s identification of three distinct 13th century glass workshops, including the master of the Ezekiel window
• The late 15th century west rose window and silver stain, which let artists paint yellow and amber directly onto clear glass
• How restorers used illuminated manuscripts to recreate the colors, and why the 1852 cedar spire is the fifth on the roof
• The 2008 restoration that cleaned the glass with lasers and added a thermoformed outer layer with a ventilated gap