Did You Know This?

According to the Harry Potter News Aggregator, the way that member of the Order of the Phoenix communicate is answered in Book 5.

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). The Patronus is an immensely efficient messenger for several reasons: it is an anti-Dark Arts device, which makes it highly resilient to interference from Dark wizards; it is not hindered by physical barriers; each Patronus is unique and distinctive, so that there is never any doubt which Order member has sent it; nobody else can conjure another person's Patronus, so there is no danger of false messages being passed between Order members; nothing conspicuous needs to be carried by the Order member to create a Patronus.

I don't remember reading this...unless I was to figure it out from the scene in Doby's room of requirement when Harry was teaching the Patronus spell. Perhaps there is an implication there that I missed out on. Now I'm going to have to re-read that book again.