The Chapelle des Reliques is the final chapel off the south side of the choir and can be traced back to the 13th century. It is accessed through a door in the Chapelle Saint Yves. Originally a courtyard, it was roofed over in the 14th century and was known as the "Petite Trésorerie". In 1897 Monseigneur Fallières transformed it into a chapel to hold relics. The ornate reliquaries held within date to the 19th century and are said to contain a piece of the actual cross ("la Vraie Croix") and a thorn from the crown of thorns. The reliquaries also contain the skull of Saint Guillaume and a relic of Saint Brieuc. The chapel is secured by an ornate iron gate.