Angel Hierarchy

The angels are grouped in a hierarchy or Choirs althought there is much speculation as to how these hierarchies or choirs are grouped. According to the Catholic tradition there are nine choirs of angels which are:
Seraphim
Cherubin
Thrones
Dominions (Authorities)
Virtues (Lordships)
Powers
Archangels
Principalities
Angels

This Christian angelic hierarchy was put forward by Pseudo-Dionysius the Areopagite in the 4th or 5th century in his book De Coelesti Hierarchia (The Celestial Hierarchy). Pseudo-Dionysius and Thomas Aquinas, in his work called Summa Theologica, drew on passages from the New Testament, specifically Ephesians 1:21 and Colossians 1:16, in an attempt to reveal a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three Orders or Choirs.
The nine choirs of angels are divided into three spheres as follows:

St. Thomas Aquinas in Summa Theologica (1225–1274):
1. Seraphim, Cherubim, and Thrones;
2. Dominations, Virtues, and Powers;
3. Principalities, Archangels, and Angels.