Two-way mirror

"This is a two-way mirror, I've got the other one of the pair. If you need to speak to me, just say my name into it; you'll appear in my mirror and I'll be able to talk in yours. James and I used to use them when we were in separate detentions."

- Excerpt from the note of Sirius Black to Harry Potter, read by Harry after Sirius' death in 1996.

A two-way mirror is one in a pair of mirrors that are magically connected. This allows people to communicate with each other between locations.

Known owners
During their years in school, James Potter and Sirius Black used a pair of two-way mirrors so they could talk to each other while they were in separate detentions. Sirius later gave James' mirror to Harry in 1996. After Sirius' death, his mirror came into the possession of Mundungus Fletcher, who in turn sold it to Aberforth Dumbledore.

Harry smashed his mirror in 1996, after determining he could not contact Sirius' spirit, since Sirius did not have his own mirror with him when he died. In 1997, before Harry left the Dursley's house, he emptied out his school trunk and cut the fourth finger of his right hand twice on a shard of the mirror. Despite the apparent uselessness of this shard, Harry kept it with him in his mokeskin pouch during his search for Voldemort's Horcruxes.

Aberforth's possession of one of the two-way mirrors became a great help to Harry and his companions in 1998. After being kidnapped by the Snatchers and brought to Malfoy Manor, Hermione was tortured by Bellatrix Lestrange, who wanted to know how Godric Gryffindor's Sword had come into their possession. After no success with his Golden Snitch and his broken phoenix wand, Harry grabbed the mirror shard and saw a gleam of bright blue eye and thought that they were Dumbledore's. He called for help, and Aberforth managed to send Dobby to rescue them.

Behind the scenes

 * While the mirror itself does not appear, Harry and his friends witness Aberforth's purchase of it from Mundungus Fletcher in Harry Potter and the Half-Blood Prince.
 * When Sirius gives Harry the mirror in a badly wrapped package, Harry vows to himself he will never use it, not knowing what it is. Ironically, if he had used it rather than Dolores Umbridge's fire to contact his godfather, Kreacher would not have been able to misinform Harry about Sirius' whereabouts, meaning Harry would never have gone to the Department of Mysteries and Sirius would not have died. This unhappy truth does not seem to occur to Harry in the novel (and the mirror is not present in the film version), though arguably, he needs no more reasons to lay blame for Sirius' death at that point. Also, there's no way of knowing if using the mirror would have helped, unless Sirius kept it on his person at all times.
 * The mirror could possibly be charmed with a Protean Charm, like the one used by Hermione on the Dumbledore's Army Galleons.
 * The two-way mirror was never seen in the films prior to Harry Potter and the Deathly Hallows: Part 1, and its origin and significance are not explained.

Appearances

 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1

Notes and references
Miroir à Double Sens