Inspired by Voldemort's Dark Mark, Hermione created fake galleons with the Protean Charm placed on them. On a real galleon, there were numerals around the edge to represent the serial number of the goblin who cast the coin. On these coins, they changed to represent the date and time of the next Dumbledore's Army meeting whenever the master coin changed. The coins also warmed to alert carriers that they changed. In 1998, Neville Longbottom used the coins to alert former DA members of the beginning of the Battle of Hogwarts to fight the Death Eaters and defend the school.

Draco Malfoy used the same concept to keep in contact with Madam Rosmerta, whom he had under the Imperius Curse.