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The workings of the fake Galleon communication system is described very specifically in Order of the Phoenix:

Hermione soon devised a very clever method of communicating the time and date of the next meeting to all the members in case they needed to change it at short notice, because it would look so suspicious if people from different Houses were seen crossing the Great Hall to talk to each other too often. She gave each of the members of the D.A. a fake Galleon (Ron became very excited when he saw the basket at first, convinced that she was actually giving out gold).

"You see the numerals around the edge of the coins?" Hermione said, holding one up for examination at the end of their fourth meeting. The coin gleamed fat and yellow in the light from the torches. “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."

This appears to be a very simple system. Each individual coin is charmed to duplicate Harry's coin. So whatever appears on Harry's coin will appear on everyone else's coin as well.

Yet later in the series we see messages being passed from one coin to another without the use of Harry's coin. At the end of Half-Blood Prince Harry tells Ron and Hermione to rouse the DA members via the coins, yet he does not give them his coin nor does he tell them where his coin is so that they could use it:

Use anyone else who you can rustle up from the D.A., Hermione, those contact Galleons will still work, right?

If Hermione changed the numbers on her own coin, it should not have transmitted any message to anyone else's coin because their coins were duplicates of Harry's coin, not hers.

Similarly, in Deathly Hallows we find Neville and Luna both using their coins to send messages:

"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 out 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.

Note that the above quote apparently claims that the D.A. had used the coins to send one another messages, something that was never mentioned in Order of the Phoenix and is certainly beyond the scope of the system Hermione described.

Also in Deathly Hallows:

"I got the message," she said, holding up her own fake Galleon, and she walked over to sit beside Michael Corner.

And:

"We're fighting, aren't we?" said Dean, taking out his fake Galleon. "The message said Harry was back, and we were going to fight! I'll have to get a wand, though —"

Once again, these are communications that were made without the use of Harry's coin.

So when and how exactly did the communication system change? How did it go from a bunch of coins being charmed to duplicate whatever message was written on Harry's coin, to the ability for anyone to communicate with anyone?

  • What makes you think the process was one-way? I would assume that all of the coins would reflect any change made to any of them. – Harry Johnston Nov 21 '18 at 7:21
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    @HarryJohnston For one, Hermione says "they’ll all change to mimic his". Additionally, if anyone could change it that would be a recipe for disorder. – Alex Nov 21 '18 at 7:24
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    If Harry changes his coin, the rest of the coins will change too. Hermione said nothing either way about what would happen if anybody else made a change to their coin, because that wasn't relevant to the plan. – Harry Johnston Nov 21 '18 at 7:28
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    @Harry My reading of the text is that it's analogous to Harry's coin being a transmitter and all the other coins are just receivers "tuned" to that frequency (like a radio broadcaster and radio sets). So at what point did the other galleons not only become transmitters but also able to transmit to/received at a specific destination. – Jon Clements Nov 21 '18 at 8:50
  • @JonClements I'd be inclined to agree, though it wasn't explicitly stated. I think it's probable that it was an afterthought by Rowling to make it work both ways. Or maybe Hermione figured out a way of altering them further to work both ways (if they brought them to the meeting, it is a perfectly viable possibility). – Neil Nov 21 '18 at 11:39
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Hermione never said Harry's coin was special in any way

Let's look at the quote you've provided:

"On these fake coins, though, the numbers will change to reflect the time and date of the next meeting... 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."

The parts I emphasized are those to make us think Harry coin was "the main coin". But it may not be the case. Hermione does not explain all the details of the Protean Charm here, she only explains how DA members will use the coins in their situation. Meaning: Harry changes the date - everyone else receives the message. There may be more to the Protean Charm, but why mention it if changing info on Harry's coin is all they need for now?

Mind that Hermione also says:

"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."

She doesn't say: "Harry will send you whatever messages he wants". That also looks like restriction to the coins' functionality. But later we see that the coins are proper sms-like way of communication.

Looks to me like Hermione in the given quote wasn't explaining how the coins worked in general. She rather gave people instructions how they will use the coins in DA. That is, Harry was the only one to be allowed to change his coin, and the changing was limited (by the agreed rules, not magic) to date for safety reasons.

  • "Numbers" is not a restriction of functionality. Galleons have serial numbers, numbers can be used to represent a date and time, so it's unobtrusive to use numbers. But how does Harry change the numbers? Clearly he uses magic to put on whatever numbers he wants. For the same price, why wouldn't he be able to put on letters? It wouldn't be necessary for scheduling DA meetings but there's no reason why it shouldn't work. – Alex Nov 22 '18 at 9:28
  • @Alex Well, when she says it, it sounds (to me at least) like you can change numbers, that is - numbers to numbers. Not letters of runes or pictures or cartoons etc... But as I said in my answer that it not the case and she said "numbers" only because that's what was needed in their situation. – Shana Tar Nov 22 '18 at 9:42
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    That was actually the original version of my question (how did it evolve from numbers to messages?) But then I realized that there’s no reason why it would have to be numbers. If you can charm the coin to say 543754686 instead of 3835264848 then presumably you can also charm it to say “I’m safe - Luna” or the like. – Alex Nov 22 '18 at 9:45
  • The difference, though, is that numbers vs letters is not a different functionality. Sending vs receiving is. – Alex Nov 22 '18 at 9:47
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It was actually Hermione, who enchanted the coins

So that perfectly explains how was she able to spread the message without Harry's original coin - I don't see a reason Hermione wouldn't be able to charm another coin.

As to coins used to communication

"We're fighting, aren't we?" said Dean, taking out his fake Galleon. "The message said Harry was back, and we were going to fight! I'll have to get a wand, though —"

As have been said by Dumbledore, news spread very quickly around the Hogwarts, so I assume people heard that Harry's back, then they discovered that coins are active and were able to do the simple math.

  • Even if Hermione could enchant another coin it wouldn't help; it's the other students' coins that need to be enchanted to mimic hers. I'm not sure what your last paragraph is arguing. Are you saying that the coins were not actually the method of communication? – Alex Nov 22 '18 at 9:25
  • No, what I'm saying in the last paragraph is that coins didn't have to send the exact message that 'Harry is back'. If former members of DA received any message (date and time, for example) by these coins while also already knowing that Harry might be back through rumours - then they could've done the simple math. – Nikita Neganov Nov 22 '18 at 10:17
  • Ah, well my question was how the coins could send any message. – Alex Nov 22 '18 at 13:00
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It is possible that the Protean charm always works this way.

I can't find any canon information on this either way. Hermione never said anything about what would or wouldn't happen if one of the other students transfigured their own coin. Nor would that be surprising, since it wasn't relevant to the purpose of the coins, and the other students would presumably know how a Protean charm works anyway, even if they didn't know how to cast it yet.

It seems to me to be more elegant for the spell to always make all the enchanted items reflect one another (many-to-many) rather than having a master item that the others reflect (one-to-many) but that's hardly conclusive. It is also thought that the Dark Mark uses the Protean charm, in which case the charm is probably many-to-many rather than one-to-many, but the canon evidence on this point isn't conclusive either.

... but even if it doesn't, Dumbledore's Army could have modified the coins.

"...Protean Charm?... But that's... that's N.E.W.T. standard, that is."

It was surprising when Hermione was able to cast the Protean charm in her fifth year, but by the time of Deathly Hallows most of the students known to be in Dumbledore's Army were studying at N.E.W.T. level, and several were in their final year. If it was indeed necessary for them to re-enchant the coins to explicitly make them many-to-many, or to change which coin was the master, that would not have been an insurmountable obstacle.

  • I'm not sure that's a conclusive interpretation of the Muggle's Guide. It also says there that the Protean Charm "Makes objects mirror a master copy", which sounds like the other coins would only be able to mimic Harry's which is the master copy. – Alex Nov 25 '18 at 22:32
  • Ah. Didn't notice that. The Wikia also talks about a master copy. I'll remove the references. – Harry Johnston Nov 25 '18 at 22:39
  • Yeah, I noticed the older versions too. As for how else to explain Deathly Hallows... that's my question. (I grant the possibility that that's how Hermione set it up all along, but the issue with that is that unless that's the only way the Protean charm can work it would seem to be a waste of effort plus adding additional risk.) – Alex Nov 25 '18 at 23:00
  • ... come to think of it, there is another possibility. I've heavily revised my answer. – Harry Johnston Nov 25 '18 at 23:35
  • I like the revised version better, but it still leaves an issue. If it is the case that the basic Protean Charm only makes the other coins imitate a specific coin (and I realize that you also argue that this is not necessarily so) then you would think that to extend the charm they would need to cast in on the receiving coins, rather than the broadcasting coins. That is to say that they would have to enchant each person's coin to also mimic Neville's coin. But some of the coinholders were not even at Hogwarts in Deathly Hallows. – Alex Nov 25 '18 at 23:43

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