"AND EITHER MUST DIE AT THE HAND OF THE OTHER FOR NEITHER CAN LIVE WHILE THE OTHER SURVIVES..."
When reading detailed studies of the prophecy on page 841 in Order of the Phoenix, this line is often the least spoke of. The only accepted meaning is the most obvious, that either Harry must die at the hand of Voldemort or Voldemort to Harry. And that very well may be the meaning, I'm not denying that. But, in my eyes, it can be read in a totally new light, and totally change what we come to expect from the next two Harry Potter installments. Here is that very same line written in a different way.
"AND EITHER HARRY OR VOLDEMORT MUST DIE AT THE HAND OF THE OTHER FOR NEITHER HARRY NOR VOLDEMORT CAN LIVE WHILE THE OTHER SURVIVES..."
What's the difference? you may ask. I hope you can already figure that out. "The Other," notice how it is mentioned twice in this same line. Now, let's throw the title of book six into the mix and it should all become obvious. The Half-Blood Prince, the so called "HBP" whom is neither Harry or Voldemort. Let's read this line one more time in the way I have come to believe it should be read.
"AND EITHER HARRY OR VOLDEMORT MUST DIE AT THE HAND OF THE HBP FOR NEITHER HARRY NOR VOLDEMORT CAN LIVE WHILE THE HBP SURVIVES..."
Now we all know by now that Rowling hides her story in the small, overlooked details. And she said herself that "she and Madam Trelawney worded the prophecy very carefully". And furthermore, read what she said when finally squashing the rumor of Mark Evans....
"Ha Ha! Yes, Mark Evans is back, suckers, and he's the key to everything! He's the Half-Blood Prince, He's Harry's Great-Aunt, He's the Heir of Gryffindor, He lives up the Pillar of Storge, and he owns the Mystic Kettle of Knackledirk." Now, obviously, this was a sarcastic comment about Mark Evans, but still, according to it, it seems like she's telling us in an offhand way that the HBP is the key to everything. As a matter of fact, it seems like everything I've read from JK Rowling supports my theory. And here it is one last time just so it'll sink in.
"AND EITHER HARRY OR VOLDEMORT MUST DIE AT THE HAND OF THE HBP FOR NEITHER
HARRY NOR VOLDEMORT CAN LIVE WHILE THE HBP SURVIVES..."