On real Galleons that's just a serial number referring to the goblin who cast the coin. On these fake coins, though, the numbers will change to reflect the time and date of the next meeting. The coins will grow hot when the date changes, so if you're carrying them in a pocket you'll be able to feel them. We take one each, and when Harry sets the date of the next meeting he'll change the numbers on his coin, and because I've put a Protean Charm on them, they'll all change to mimic his.
-- Hermione Granger to Dumbledore's Army (OP19)
Complex spell that makes something change form.
References from the canon
- Hermione was able to cast a Protean Charm on the fake Galleons the D.A. used during the 1995 - 1996 [Y15 - Y16] school year to pass along the date and time of each meeting. The spell caused the numbers on the members' coins to change when Harry changed the numbers on his coin. The Ravenclaws were all astounded that Hermione, who wasn't in their house, could cast this spell, since it's N.E.W.T. level. Clearly none of them could cast this magic at fifth year level (OP19).
- Ironically, Hermione got the idea from Voldemort's use of the Death Eaters' Dark Marks as a means of communication.
- Draco Malfoy, in turn, deliberately copied the notion of Hermione's D.A. coins to enchant coins as a means of communication for himself with Madam Rosmerta (HBP27).
"protean" Eng. able to readily assume a different form, from Proteus, a sea god from Greek mythology who could change his shape rapidly