They say three's a magic number, and it is. Seven is the most powerful, of course, but three is the binding, again and again: the only equal binding there is.
There's one exception, but they're the Founders: it makes sense that they'd be special. In any case, that secret is lost to the mists of time.
There are no such things as spies, in the wizarding world. How could there be? So it's an impossibility, when Dumbledore says, "Severus worked for me."
But Dumbledore has worked impossibilities before, and the Fidelar spell shows no loyalty at all. Everyone knows that Dumbledore never binds his followers; modern foolery, they sneer.
When Dumbledore dies, his followers fight on.
It's an accident, their binding. None of them meant for it. None of them realize it until Harry, looking in the mirror, sees that one of his hairs has grown in red.
He thinks of Dumbledore, and cries.
In a world where everyone is loyal, the Hufflepuffs flock to Dumbledore's banner. Voldemort never wonders why that is, but Hermione does.
He comes to them one night, pale with shock, and tells them what they already know. The secret's hidden in the castle's independence, if only anyone would listen for it.
Harry thinks about Voldemort's death, pictures it. There aren't many powerful enough to hold a following, and he wonders what will happen to Voldemort's, when he dies.
It's the only way, they say. He doesn't give a damn. Dumbledore didn't, and neither will he.
Dumbledore told him, once, about language and its shiftings. Things can change very easily, Harry, he said. Did you know that Merlin's beard was once Merlin speared? A pause. I thought not.
When Harry finds out about the Deathly Hallows, he wonders. Looks at the wand, its shape: energy concentrated to a point, Hermione says.
He shakes his head, and moves on.
Harry feels the binding close around him, and knows himself the weaker for it.
He wonders, though, about Dumbledore. Trust is a choice, he'd said. Now Hermione tells him: choose again.
He does.
The choice is still the same.