Muggles' Guide to Harry Potter/Magic/Protean Charm

Overview
The Protean Charm makes multiple objects act as if they are the same object – changes made to one object appear on all of them.

Extended Description
In order to make it easier to contact all the members of Dumbledore's Army, Hermione creates a large number of false Galleons. These Galleons are charmed such that, once the date of the next meeting is determined, Harry can change one Galleon to show that date, and all the others will similarly change, and heat up to let their holders know that they have changed. Terry Boot recognizes the charm as a Protean Charm, and is impressed: this is NEWT-level magic.

A parallel is drawn to the Dark Mark tattoo that Death Eaters wear on their left forearm, and Hermione indicates that she had not thought that Harry would want to inflict pain on his prospective students.