The magical keys used by Celestial Wizards in the Fairy Tail series.
Gate keys are magical items that are necessary to summon Celestial Spirits. They function to open the gate between the celestial spirit world to the human world in order for the Celestial Spirit to pass through and help aid their master in battle. Each spirit has their respective key, with its own unique design, that can be used to call upon it when needed. But, depending on the spirit, certain requirements need to be met in order to summon him or her and/or must be summoned on the days that the master and spirit had agreed to in their contract. However, some spirits can pass through the gate without a key, such as Loke and Virgo, but doing so means that they must use their own magical powers rather than those of their master.
Currently, gate keys can be classified into two classes: the more common keys are Silver Keys while Gold Keys are of greater rarity and are represented by the 12 signs of the Zodiac. It is still unknown as to whether there are more different types of gate keys.