The Sainte-Chapelle or "Holy Chapel", in the courtyard of the royal palace on the Île de la Cité (now part of a later administrative complex known as La Conciergerie), was built to house Louis IX's collection of relics of Christ, which included the Crown of Thorns, the Image of Edessa and some thirty other items. Louis purchased his Passion relics from Baldwin II, the Latin emperor at Constantinople, for the sum of 135,000 livres, though this money was actually paid to the Venetians, to whom the relics had been pawned. The relics arrived in Paris in August 1239, carried from Venice by two Dominican friars. Upon arrival, King Louis hosted a week-long celebratory reception for the relics. For the final stage of their journey they were carried by the King himself, barefoot and dressed as a penitent, a scene depicted in the Relics of the Passion window on the south side of the chapel. The relics were stored in a large and elaborate silver chest, the Grand-Chasse, on which Louis spent a further 100,000 livres. The entire chapel, by contrast, cost 40,000 livres to build and glaze. Until it was completed in 1248, the relics were housed at chapels at the Château de Vincennes and a specially built chapel at the Château de Saint-Germain-en-Laye. In 1246, fragments of the True Cross and the Holy Lance were added to Louis' collection, along with other relics. The chapel was consecrated on 26 April 1248 and Louis' relics were moved to their new home with great ceremony. In addition to serving as a place of worship, the Sainte-Chapelle played an important role in the political and cultural ambitions of King Louis and his successors. With the imperial throne at Constantinople occupied by a mere Count of Flanders and with the Holy Roman Empire in uneasy disarray, Louis' artistic and architectural patronage helped to position him as the central monarch of western Christendom, the Sainte-Chapelle fitting into a long tradition of prestigious palace chapels. Just as the Emperor could pass privately from his palace into the Hagia Sophia in Constantinople, so now Louis could pass directly from his palace into the Sainte-Chapelle. More importantly, the two-story palace chapel had obvious similarities to Charlemagne's palatine chapel at Aachen (built 792–805)—a parallel that Louis was keen to exploit in presenting himself as a worthy successor to the first Holy Roman Emperor.
The contemporary visitor entering the courtyard of the Royal Palace would have been met by the sight of a grand ceremonial staircase (the Grands Degres) to their right and the north flank and eastern apse of the Sainte-Chapelle to their left. The chapel exterior shows many of the typical characteristics of Rayonnant architecture—deep buttresses surmounted by pinnacles, crocketted gables around the roof-line and vast windows subdivided by bar tracery. The internal division into upper and lower chapels is clearly marked on the outside by a string-course, the lower walls pierced by smaller windows with a distinctive spherical triangle shape. Despite its decoration, the exterior is relatively simple and austere, devoid of flying buttresses or major sculpture and giving little hint of the richness within. No designer-builder is named in the archives concerned with the construction. In the 19th century it was assumed (as with so many buildings of medieval Paris) to be the work of the master mason Pierre de Montreuil, who worked on the remodelling of the Royal Abbey of Saint-Denis and completed the south transept façade of Notre-Dame Cathedral in Paris. Modern scholarship rejects this attribution in favour of Jean de Chelles or Thomas de Cormont, while Robert Branner saw in the design the hand of an unidentified master mason from Amiens.The Sainte-Chapelle's most obvious architectural precursors include the apsidal chapels of Amiens Cathedral, which it resembles in its general form, and the Bishop's Chapel (c. 1180s) of Noyon Cathedral, from which it borrowed the two-story design. As has often been argued however the major influence on its overall design seems to have come from contemporary metalwork, particularly the precious shrines and reliquaries made by Mosan goldsmiths.Metal also appears to have been used in the structure. Though the buttresses are substantial, they are too close to the vault to counter its side thrust. Metal elements such as iron rods or chains, able to support tension, must have been used to replace the flying buttresses of previous structures.
The Parisian palatine chapel, built to house a reliquary, was itself like a precious reliquary turned inside out (with the richest decoration on the inside). Although the interior is dominated by the stained glass (see below), every inch of the remaining wall surface and the vault was also richly coloured and decorated. Analysis of remaining paint fragments reveals that the original colours were much brighter than those favoured by the 19th-century restorers and would have been closer to the colours of the stained glass. The quatrefoils of the dado arcade were painted with scenes of saints and martyrs and inset with painted and gilded glass, emulating Limoges enamels, while rich textiles hangings added to the richness of the interior. Above the dado level, mounted on the clustered shafts that separate the great windows, are twelve larger-than-life-sized sculpted stone figures representing the twelve Apostles (six of these are replicas—the damaged originals are now in the Musée du Moyen Age). Each carries a disk marked with the consecration crosses that were traditionally marked on the pillars of a church at its consecration. Niches on the north and south sides of the chapel are the private oratories of the king and of his mother, Blanche of Castile.
The most famous features of the chapel, among the finest of their type in the world, are the great stained-glass windows, for whose benefit the stone wall surface is reduced to little more than a delicate framework. Fifteen huge mid-13th-century windows fill the nave and apse, while a large rose window with Flamboyant tracery (added to the upper chapel c. 1490) dominates the western wall. Despite some damage the windows display a clear iconographical programme. The three windows of the eastern apse illustrate the New Testament, featuring scenes of The Passion (centre) with the Infancy of Christ (left) and the Life of John the Evangelist (right). By contrast, the windows of the nave are dominated by Old Testament exemplars of ideal kingship/queenship in an obvious nod to their royal patrons. The cycle starts at the western bay of the north wall with scenes from the Book of Genesis (heavily restored). The next ten windows of the nave follow clockwise with scenes from Exodus, Joseph, Numbers/Leviticus, Joshua/Deuteronomy, Judges, (moving to the south wall) Jeremiah/Tobias, Judith/Job, Esther, David and the Book of Kings. The final window, occupying the westernmost bay of the south wall brings this narrative of sacral kingship right up to date with a series of scenes showing the rediscovery of Christ's relics, the miracles they performed, and their relocation to Paris in the hands of King Louis himself.