In literature, canon refers to source material that establishes the characters, relationships, events, and environment of a fictional universe. For Harry Potter fans, that includes the geographic location of Hogwarts (Scotland), the necessary gestures to perform a Levitation Charm (“swish and flick”), and the official rules for a game of Quidditch. Outside of canon, however, exists headcanon—or fans’ imagined explanations for what happens between the lines, or off the page entirely. While some of these speculations are truly wild, certain fan theories come closer to the mark than others. To the astute Harry Potter fans who saw these coming, we salute you.
Harry’s Invisibility Cloak was more than meets the eye.
Even in a world where owls deliver mail and sporting events occur mid-air, there’s something particularly magical about an invisibility cloak. Harry was suitably flabbergasted when he found one waiting under the Christmas tree for him in his first year at Hogwarts, but he had no idea that his family heirloom wasn’t just rare, but one-of-a-kind. When keen readers of Fantastic Beasts and Where to Find Them learned that invisibility cloaks are most commonly woven from Demiguise hair, they may have thought it strange that Harry’s cloak was never described as being, well, hairy. Knowing that even the strongest enchantments placed on a normal garment would wear off over time, clever fans started to suspect there was more to learn about James Potter’s bequest to his son. The shoe dropped in Deathly Hallows, when Ron realized that Harry’s cloak was none other than Ignotus Peverell’s fabled Cloak of Invisibility—one of the three Hallows, and one hell of a hand-me-down.
Dumbledore and Grindelwald were more than just friends.
Fans are the first to admit that their unconventional ships (short for “relationships”) are more wishful thinking than predictions, but every once in a while, their dreams do come true. Having learned in Deathly Hallows that a young Albus Dumbledore had made fast friends with Gellert Grindelwald in their impressionable teen years, a tiny niche of the internet began to wonder if the two might have been romantically involved.
In 2007, Rowling revealed to a Carnegie Hall audience that not only was the late Headmaster gay, but that he had in fact fallen "in love with Grindelwald." Whether his romantic feelings might have clouded his judgment of the most powerful Dark wizard of his time or not is a secret Dumbledore took to his grave, but there's no question that having to take down your best friend would be enough to haunt someone for a lifetime.
Magic is genetic.
When Harry steps through the brick wall behind The Leaky Cauldron into Diagon Alley for the first time, he crosses a physical boundary that divides two worlds: Muggle and magical. The coexistence of these two types of people raised an obvious question for early readers of the series: What makes some people magical and others not?
In a 1999 interview, Rowling claimed that “Nobody knows where magic comes from. It is like any other talent. Sometimes it seems to be inherited, but others are the only ones in their family who have the ability.” Fans demanded a better explanation than a shrug, and it fell to the scientists among them to come up with a satisfying solution. Certain wizards’ obsession with remaining “pure-blood,” meaning having only magical ancestors and no Muggle relations, tipped off fans to the idea that magic might be something literally in someone’s blood—that is, in their DNA. Heated debates about the feasibility of this theory broke out; Punnett squares were deployed.
In 2012, Rowling explicitly stated that Squibs, or non-magical people born to magical parents, are rare, because “magic is a dominant and resilient gene.” She later complicated things, however, by claiming that Muggle-borns like Hermione and Lily Potter are possible because of “recessive genes”—a statement that was in direct contradiction to her earlier attempt to reconcile magic and science. Saving those of us who hadn’t thought about chromosomes since sophomore year of high school, avid fan and biology major Andrea stepped in to explain (with a six-page paper!) that magical ability could be attributed to “a single autosomal dominant gene if it is caused by an expansion of trinucleotide repeats with non-Mendelian ratios of inheritance.” Five years later, Duke University professor Eric Spana further explained that Muggle-borns like Hermione and Lily could be accounted for by de novo mutations—basically, winning the genetic lottery.
Harry and Ginny had good reason for not naming their sons after Remus.
Fans have had a field day with Harry and Ginny’s, er, unusual naming system for their children, which honored a number of late and living loved ones, but seemed to be conspicuously absent one name. With James Sirius representing two of the three Marauders who didn’t turn out to be cowardly traitors, it would naturally fall to the Potters’ second son to carry on the legacy of the man who taught Harry how to defend himself against Dementors and gave his life to help defeat the Death Eaters. The fact that the middle Potter child was named Albus Severus after Dumbledore and Snape rather than Lupin outraged a number of fans, who were quick to point out that Remus is a lovely name, and certainly one less fraught with baggage than that of the former Potions Master. An optimistic few, however, rushed to find an explanation: Perhaps the Potters hadn’t skipped over Remus Lupin, but saved his memory for someone else. Rowling affirmed this act of kindness over Twitter, saying that “Harry left Remus’s name for Teddy to use for his own son”—not a snub, but a gift.
Harry dies! / Harry lives!
One of the hottest prediction games in town prior to the release of Deathly Hallows was a question of literal life or death: whether Harry Potter would remain The Boy Who Lived, or be remembered forever after encountering Voldemort one final time as The Boy Who Didn’t Quite Make It. The prophecy that caused Voldemort to make an attempt on baby Harry’s life all those years ago stated quite clearly that "either must die at the hand of the other for neither can live while the other survives," meaning either Harry or Voldemort had to die. There were strong arguments on both sides, largely distilled into, “The main character of a children’s book series can’t be killed by the villain; what kind of message does that send?” vs. “Wouldn’t it be wild if that did happen, though?”
The twist, of course, is that there was a third option. In a rare case where both sides were right—sort of—Harry did cross into the afterlife, but made an unprecedented return. While nearly every reader could thus be satisfied in claiming that they were right (again, sort of) the surprise spelled a costly payout for London-based bookies William Hill, who had taken bets on Harry’s survival odds. "Since the book came out, it's been subject to quite a few interpretations," a spokesperson said. "So we paid out on all the bets," to the tune of £62,000 (or just over $100,000 in today's dollars).