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 magicallyconnected. This allows people to communicate with each other whilst in different locations.

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 sold it to Aberforth Dumbledore.

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. Experiencing 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 it was Dumbledore's. He called for help, and Aberforth managed to sendDobby to rescue them.