Mirror bridges were created by Magicians as an alternative to traveling through the Mirror Realm due to the dangerous and unpredictable nature of magic in the dimension. They are also used by Hedge Witches to travel between safe houses.
In preparation for their heist in the Library of the Neitherlands, Poppy Kline stole the Alice Quinn's notes from her time as a Niffin for Quentin to study. Using Quinn's notes, Coldwater and Kline decided that to use Victoria Gradley's blood to build a mirror bridge to the Satellite Library to retrieve one of the Seven Golden Keys that Penny was sending from the Library of the Neitherlands Underworld Branch.
Mark, a hedge witch previously arrested by Sam Cunningham, used a mirror bridge to escape from the detective. Later, Marina Andrieski brought Cunningham, along with Issac Karamov, Janet Pluchinsky, and DJ Hansel, to her penthouse to remove their Glamours.
Zelda Schiff recruited Gavin to open a mirror bridge into the Mirror Realm in order to rescue her daughter, Harriet, who was trapped there by Gavin during the former's attempt to expose the Order for using fairy dust.
The Mirror Bridge is an incredibly difficult and delicate spell that requires a skill level equal to that of a Master Magician, as well as a large amount of Traveller blood. The spell, if done incorrectly, would result not only in the death of the Traveller but anyone attempting to use the Bridge. The delicate nature of the spell involves the metamathematics involved in calculating the distance between locations.
The Bridge is formed by Travellers drawing a sigil onto a mirror in their blood, which must be fed regularly with fresh blood, proving to be increasingly dangerous for the Traveller as the blood must be fresh. The Bridge can be constructed inadequately, leaving a chilling aftereffect on the user.
- Mirror Bridges were first featured in The Magician King when Julia and Quentin were traveling between safe houses, but they were never named.