Celestial Spirit Gate Keys are magical keys that can be used to summon Celestial Spirits, beings from another dimension that can be used to fight their contractor's opponents, to do chores or just to hang around with. There are two different types of Celestial Spirit Gate Keys: the Silver Keys and the Gold Keys. The Silver Keys are common keys that can be bought from stores while the Gold Keys are very rare keys that can be used to summon the Zodiac Spirits. According to Yukino, it is said that when all twelve Zodiac Keys are gathered, one is able to "open the gate to changing the world".
It is possible for a Celestial Spirit Mage to summon the Celestial Spirit King, who does not have a physical key, but there are certain conditions to do so. The first condition is the ability to summon at least three Spirits at once. The second is that a Golden Zodiac Key must be sacrificed to do so. The final condition is that the Mage and the Spirit of the Key must have a high level of trust between them. Whichever Key is sacrificed to summon the King will then crumble to pieces, and will henceforth be unusable. As such, the Spirit of the Key will no longer be able to travel the Mage's world. Therefore doing so is considered the very last resort.