Celestial Spirits are Magical beings from their own separate universe, the Celestial Spirit World. Some Mages, called Celestial Spirit Mages due to their specialization in Celestial Spirit Magic, are able to summon Spirits by opening their Gates via the use of Celestial Spirit Gate Keys. These Keys are separated into two classes: the more common Silver Keys and the extremely rare Gold Keys. Keys for Celestial Spirits are counted in Units (collectively without regard of Gold or Silver), and a Celestial Spirit Mage is noted by how many Units they have obtained. All Celestial Spirits are each named after a different astronomical constellation. Out of the twelve Gold Keys, ten are owned by Lucy Heartfilia and the remaining two are owned by Yukino Aguria, who also holds the key to the "13th Gate" of the Zodiac.