A hierarchy of angels is a belief or tradition found in the angelology of different religions, which holds that there are different levels or ranks of angels. Higher ranks may be asserted to have greater power or authority over lower ranks, and with different ranks having differences in appearance, such as varying numbers of wings or faces.
The Jewish angelic hierarchy is established in the Hebrew Bible, Talmud, Rabbinic literature, and traditional Jewish liturgy. They are categorized in different hierarchies proposed by various theologians. For example, Maimonides, in his Mishneh Torah or Yad ha-Chazakah: Yesodei ha-Torah, counts ten ranks of angels.
The most influential Christian angelic hierarchy was that put forward by Pseudo-Dionysius the Areopagite in the 4th or 5th century in his book De Coelesti Hierarchia (On the Celestial Hierarchy). During the Middle Ages, many schemes were proposed, some drawing on and expanding on Pseudo-Dionysius, others suggesting completely different classifications. According to medieval Christian theologians, the angels are organized into several orders, or "Angelic Choirs".
Pseudo-Dionysius (On the Celestial Hierarchy) and Thomas Aquinas (Summa Theologica) drew on passages from the New Testament, specifically Ephesians 1:21 and Colossians 1:16, to develop a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three Orders or Choirs.