Louis IX (later Saint Louis) purchased the Crown of Thorns from the Latin Emperor Baldwin II in 1239, and pieces of the True Cross in the years that followed — at a price roughly half the annual budget of the French crown. A reliquary was needed for these relics, but one at the scale of a building. Sainte-Chapelle was designed for exactly that function: not a church but a walkable reliquary.

The geometry of Gothic architecture — pointed arch, ribbed vault, external flying buttress — transfers the load from the wall to a skeleton. Sainte-Chapelle pushes this possibility to its limit. In the upper chapel the structural role of the wall is near zero; the 15-metre stained-glass windows fill roughly three-quarters of the surface. The interior is not a stone box but a box filled with coloured light.

The iconographic programme of the windows tells the biblical story in 1,113 scenes, from Genesis to the arrival of the relics in Paris. The cobalt blue, manganese purple, and ruby red glass were made with techniques developed at Saint-Denis in the previous century; light passing through is both coloured and softened — this is the direct visual correlate of the central Gothic theological idea of divine light (lux divina). Abbé Suger's claim, written for Saint-Denis, that 'matter is dematerialised by light,' is here realised architecturally.

The building was partly destroyed and the windows were dismounted during the French Revolution; before being stored some glass was lost. A careful nineteenth-century restoration led by Eugène Viollet-le-Duc returned about two-thirds of the original glass to its place. The site was inscribed on the UNESCO World Heritage List in 1991. Stepping inside on a bright day, the stone frame seems largely to have erased itself — what remains is coloured, fluid, untouchable space.