The Enchanted Coins are a set of counterfeit wizarding money, specifically designed to look like large, fake, yellow-metal Galleons. Instead of the serial number typically found on a real Galleon, the coins have a series of numerals around their edge. These numbers are magically enchanted to change to convey a message, primarily the date and time of a future event.
The core of the coins' magic is a Protean Charm, a highly advanced piece of magic that links objects together. As Hermione Granger explains, this is typically considered N.E.W.T.-level magic, a testament to her considerable magical skill. When the master coin, held by Hermione, is modified, every other coin in the set instantly changes to reflect the alteration. The primary function of the coins was to serve as a discreet communication system. Hermione would change the numbers on her coin to announce the date and time of the next Dumbledore's Army meeting. To ensure the message was noticed, the coin would heat up whenever a new message was set, alerting its owner to check it. Hermione revealed she was inspired to create the coins by the way Lord Voldemort uses the Dark Mark to summon his Death Eaters. The coins function on a similar principle but were created for the purpose of rebellion and learning, providing a clever and righteous parallel to the Dark Arts method.
Hermione Granger created the set of Enchanted Coins in the autumn of 1995, during her fifth year at Hogwarts. She developed the system in response to the oppressive regime of High Inquisitor Dolores Umbridge, who, under the authority of the Ministry of Magic, had outlawed all unapproved student groups and was monitoring traditional lines of communication like the Owl Post. Upon signing the enchanted parchment to join Dumbledore's Army, each member was given one of these coins. The system proved to be highly effective and secure, allowing the D.A. to schedule their secret meetings in the Room of Requirement for several months without being discovered by Umbridge or her Inquisitorial Squad.
The Enchanted Coins are a crucial plot device in Harry Potter and the Order of the Phoenix, as they are the sole reason Dumbledore's Army could form and operate in secret. They represent the resourcefulness and resilience of the students in the face of tyranny and stand as a powerful symbol of Hermione Granger's intellect and magical prowess. The coins' utility extends beyond their initial purpose. In Harry Potter and the Half-Blood Prince, Hermione and Ron Weasley use them to summon D.A. members to combat the Death Eaters who invade Hogwarts during the Battle of the Astronomy Tower. Later, in Harry Potter and the Deathly Hallows, Neville Longbottom uses his coin to rally the D.A. when Harry Potter returns to Hogwarts, demonstrating the enduring strength and communication network that Hermione's invention provided for the final Battle of Hogwarts.
In the film adaptation of Harry Potter and the Order of the Phoenix, the coins are shown visually, and a scene depicts Harry Potter's coin heating up in his hand to alert him of a new D.A. meeting time, consistent with the book's description. (film)