“Neville, she’s all right, we’ve seen her —”
“Yeah, I know, she managed to get a message to me.”
From his pocket he pulled a golden coin, and Harry recognized it as one of the fake Galleons that Dumbledore’s Army had used to send one another messages.
“These have been great,” said Neville, beaming at Hermione. “The Carrows never rumbled how we were communicating, it drove them mad. We used to sneak out at night and put graffiti on the walls: Dumbledore’s Army, Still recruiting, stuff like that. Snape hated it.”
Harry Potter and the Deathly Hallows – J.K. Rowling.