Saint Denis, the first bishop of Paris, was beheaded in that city in the year 250. According to legend, angels accompanied him as he carried his own head from the place of execution to his chosen burial site, where later the church of Saint Denis was built just outside of Paris. Dressed in his bishop's garments, the saint is depicted with two haloes--one behind his decapitated head and a second behind the stump of his neck. This feature reflects medieval notions about the incorruptibility of saintly remains--the body does not decompose after death.
By including two haloes, the artist emphasizes Saint Denis's presence in each part of his body and the sanctity of his physical remains.