A secret Defense Against the Dark Arts group, which according to Educational Decree no. Twenty-Four is illegal but still flourished without Professor Dolores Jane Umbridge's knowledge and was termed The Dumbledore's Army and was Harry's pride. And they selected the name Dumbledore's Army - The D.A. for short because it was the Ministry's worst fear.

The members contact each other through fake galleons. When Harry changes the numerals on his own to set their next meeting's date (carefully planned so as not to clash with the houses' quidditch practices), the numerals on the members' coins automatically set themselves to match it. They can feel the coin growing hot when the numerals change. This reminds Harry of the Death Eaters' Dark Mark, and Hermione admits to have borrowed the idea, but also informs Harry that she decided to engrave the numerals on fake coins rather than the members' skins!