Traditions say she died a natural death, but accounts of her martyrdom can be found. Petronilla is thought to have been Aurelia Petronilla, a scion of the gens Flavius, the family of Vespasian and Domitian. She was also related to St. Domitilla, who was exiled in I Century to Pandateria, whose property on the Via Ardentina became a catacomb cemetary. Inscriptions there describe Petronilla as a martyr. During the papacy of Siricius (384-399), a basilica was built on the site of her tomb. In the VIII Century, Gregory III established a place of public prayer in the basilica, and her relics were translated to St. Peter's, where a chapel was dedicated in her honor. Charlemagne (d. 814) and Carlomen (d. 771) were considered adopted sons of St. Peter, and they, along with the French monarchs who succeeded them, considered Petronilla their sister.
Her chapel became the chapel of the kings of France. Her emblem, like that of St. Peter, is a set of keys.
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