JK Rowling updated her site with the answer to the
FAQ Poll question"So HOW do members of the Order of the Phoenix communicate with each other?".
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I was surprised that this particular question won the poll, because the answer (as I've already said) can be found in an already-published book (Goblet of Fire), whereas the other two questions related to book six.
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Members of the Order use their Patronuses to communicate with each other. They are the only wizards who know how to use their spirit guardians in this way and they have been taught to do so by Dumbledore (he invented this method of communication).
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And, as many of you have deduced, Dumbledore's Patronus is indeed a phoenix.
She also says we'll find out the answers to the other two questions when we read '
Half-Blood Prince'.
Source:
JK Rowling