“A similar, though less elaborate nine-fold celestial hierarchy was proposed by Pope Gregory I in his Gospel Homilies and in his Moralia on Job by collecting the Old Testament and Pauline references to angelic orders and arranging them by logical inference.
We speak of nine orders of Angels, because we know, by the testimony of Holy Scripture, that there are the following: Angels, Archangels, Virtues, Powers, Principalities, Dominations, Thrones, Cherumbim and Seraphim. Nearly every page of Scripture is witness to the fact that there are Angels and Archangels. The prophetic books, as has been noted often, speak of Cherubim and Seraphim. Four more orders are enumerated by Paul the Apostle, writing to the Ephesians, when he says, ‘Above every Principality and Power and Virtue and Domination.’ And again writing to the Colossians, he says, ‘Whether Thrones, or Powers, or Principalities, or Dominations.’ When, then, we add the Thrones to those he mentions to the Ephesians, there are five orders, to which are to be added Angels, Archangels, Cherubim and Seraphim, certainly making nine orders of Angels in all.
Homily XXIV — Pope St. Gregory I” [via]