"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."
-- Sirius Black
The two-way mirrors belong to Sirius Black. He and James used to use them when they were in separate detentions (OP38). They are a pair of small, square, old-looking mirrors (OP38). Sirius gives one of them to Harry at the end of Christmas break. He tells Harry it’s a “way of letting me know if Snape’s giving you a hard time” (OP24). Unbeknownst to Harry, Albus Dumbledore gives the other mirror to his brother Aberforth, so he can use it to watch over Harry. Aberforth uses the mirror to send help to Harry when he is trapped at Malfoy Manor (DH23).
"I want you to take this,' he said quietly, thrusting a badly wrapped package roughly the size of a package book into Harry's hands. 'What is it?' Harry asked. 'A way of letting me know if Snape's giving you hard time. No, don't open it in here!' said Sirius" (OP24).
"'OK,' said Harry, stowing the package away in the inside pocket of his jacket, but he knew he would never use whatever it was. It would not be he, Harry, who lured Sirius from his place of safety" (OP24).
"He turned the mirror over. There on the reverse side was a scribbled note from Sirius.
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.
Harry's heart began to race. He remembered seeing his dead parents in the Mirror of Erised four years ago" (OP38).
"Harry remained quite still for a moment, then hurled the mirror back into the truck where it shattered. He had been convinced, for a whole, shining minute, that he was going to see Sirius, talk to him again..." (OP38).