The nine choirs
Catholic tradition holds that angels are arranged in nine orders: Seraphim, Cherubim, Thrones, Dominations, Powers, Virtues, Principalities, Archangels, and Angels, each cited to scripture passages such as Isaiah, Genesis, Colossians, Ephesians, and Romans. The Seraphim are considered closest to God, with the ordinary Angels closest to humanity.
Origin of the devotion
The chaplet's history traces to a vision reported by a devout woman named Antonia d'Astonac, in which Saint Michael asked to be honored with nine salutations, one to each choir. Those who practiced the devotion, tradition says, were promised the escort of nine angels at Holy Communion, and those who prayed it daily were promised Michael's continual protection in life and intercession after death.
In practice the chaplet pairs each of the nine salutations with the Our Father and three Hail Marys, closes with a prayer to the four leading angels — Michael, Gabriel, Raphael, and one's guardian angel — and ends with concluding prayers addressed to Michael as protector of the Church.