The House of Gaunt
Chapter 10 of Harry Potter and the Half-Blood Prince: The House of Gaunt
Over the the next week, Hermione remains angry about Harry using the Prince's notes. She suggests that the "Prince" may be female, based on the book's handwriting, but Harry retorts, "How many girls have been Princes?" Ron is also angered, because even though Harry has offered to share, Ron is unable to read the tiny handwriting and has to rely on textbook formulas. Professor Slughorn, of course, believes Harry is among the best Potions students he has ever taught.
After dodging a tipsy Professor Trelawney in the hallway, Harry arrives at Professor Dumbledore's office for his first lesson. Dumbledore explains that they will try to determine Voldemort's intentions, based on Dumbledore's previous investigation. Dumbledore has shared all he knows with Harry; now they are embarking on guesswork and supposition. Dumbledore produces a vial containing memories from Bob Ogden, who worked in Magical Law Enforcement. Dumbledore pours the contents into his Pensieve, and he and Harry enter.
Near Little Hangleton village, Bob Ogden approaches a ramshackle cottage. A young wizard with several missing teeth and small, dark eyes staring in opposite directions appears and challenges Ogden, speaking in Parseltongue. Ogden is jinxed before stating his business. An older wizard appears and addresses the younger one as Morfin. He identifies himself as Mr. Gaunt. When Ogden says he is there about Morfin, Gaunt invites him in. Inside, Merope, Gaunt's teen-aged daughter, is in the kitchen. She has a plain, heavy face, and, like Morfin, her eyes stare in opposite directions.
Ogden says Morfin jinxed a Muggle, whereupon Merope drops a pot, causing Gaunt to taunt her. Ogden, saying Morfin has broken Wizarding law, produces a summons. Gaunt shows him a ring that he says bears the Peverell coat of arms and also points out Merope's locket, claiming it is Salazar Slytherin's, saying they are his descendants. Ogden dismisses this, their heritage has no bearing on Morfin Jinxing a Muggle. Horses are heard approaching outside. In Parseltongue, Morfin says it is the Muggle that Merope fancies and the one he Jinxed. Gaunt demands to know if Merope is chasing a Muggle. When she does not answer, he attacks her; Ogden defends her and is attacked in turn by Morfin, but Ogden escapes. Harry and Dumbledore exit the memory.
Dumbledore says Ogden quickly returned with Ministry reinforcements; Morfin was sentenced to Azkaban for three years while Marvolo Gaunt received six months. Harry instantly recognizes the name: Marvolo was Voldemort's grandfather. Dumbledore says that Merope was Voldemort's mother, and his father was the elegant Muggle on horseback that Morfin attacked. Harry wonders how the handsome Tom Riddle could possibly have married the unattractive Merope; Dumbledore suspects she used a love potion. The two ran away together, causing a scandal; Marvolo never forgave her and died shortly after his release from Azkaban.
A few months later, Riddle returned to his manor house alone, saying he had been hoodwinked. Dumbledore suggests that either Merope believed bearing Tom's child would bind him to her and stopped using the love potion or else she lost her ability to perform magic. Voldemort was raised in an orphanage when Merope died soon after giving birth.
Dumbledore agrees that Harry should share this with Ron and Hermione, but warns against spreading it any further. Harry notices a cracked ring on a table, the same ring Dumbledore wore when they visited Slughorn, and the same ring Marvolo Gaunt was wearing in the memory. Dumbledore says he acquired it recently, but turns aside further questions, and sends Harry to bed.
If another student was using the Prince's textbook, Hermione, who is a Prefect, would likely have confiscated it and reported them. However, she says nothing about Harry, and instead voices her disapproval directly to him, though she knows it will have little effect. This is a typical behavioral pattern for Hermione, who only once has reported Harry for something. In Harry Potter and the Prisoner of Azkaban, she informed Professor McGonagall that Harry had received a Firebolt broom from an anonymous donor (who she suspected was Sirius Black, then believed to be trying to murder Harry). Hermione's motivation was genuine concern over Harry's safety rather than reporting him for mischief. Unfortunately, her actions resulted in a severe backlash and being shunned by Harry and Ron for months, causing her much anguish. Hermione staunchly believes in obeying school rules, but she fears Harry and Ron's retaliation even more, and she frequently overlooks their minor misconduct. And while Hermione remains disapproving over Harry using the Half-Blood Prince's notes, and resents Harry outperforming her in Potions class, she remains silent, likely fearing another reprisal. Over time, she actually becomes quite curious about the Prince's identity. This has also subtly changed the Trio's relationship. Ron and Harry have usually sought out Hermione mostly for help or information, though they occasionally had to suffer her disdain for their academic laziness. Now Harry has another avenue for finding the information he needs, at least regarding Potions.
Dumbledore's private lessons are hardly what Harry expected. Rather than learning new magic, he is to study Voldemort's past by viewing other people's memories. Exactly how this can aid Harry is still unknown, but learning more about his enemy may help defeat him. Clues about Voldemort's own personality has been gleaned from studying the Gaunts, who had sunk to a low social status and lived in poverty after previous generations squandered the family fortune. Regardless, they still considered themselves superior to other wizard clans based solely on their bloodline's descent from Salazar Slytherin, and therefore expected preferential treatment. This reflects Voldemort's own attitude. And while the Gaunts apparently felt little need to educate or cultivate themselves, the sad, pathetic (and physically unattractive) Merope secretly yearned for a better life, despite her severely limited prospects. When she fell in love with the Muggle, Tom Riddle, she magically entrapped him to escape her abusive family and abysmal life. Unfortunately, as soon as the enchantment wore off, Riddle abandoned the pregnant Merope, who, deeply despondent and either unable or unwilling to use magic to care for herself, died soon after giving birth, leaving her son, Tom Marvolo Riddle (Voldemort), to be raised in an orphanage. While Harry, and readers, may see Merope as a sympathetic character, it should be remembered that she magically entrapped Tom Riddle, Sr., who otherwise would never have had any interest in her. It is unknown why Merope stopped using magic on him, though perhaps it becomes less effective over time, she was affected by pregnancy, or she may have hoped that Riddle would eventually fall in love with her on his own. While Riddle had every right to leave the bogus relationship, if he knew Merope was pregnant, his abandoning an innocent child is inexcusable; Merope may not have revealed her condition before Riddle was gone, though the orphanage where Tom Jr. was raised certainly knew Riddle Sr.'s identity; it is never known if they were ever in contact with him about his son, however.
- Why does Dumbledore want Harry to learn about Voldemort's past? How will it help him?
- Why would Morfin jinx a Muggle?
- What might cause Merope to lose her ability to perform magic?
Further Study 
- Hermione, a Gryffindor Prefect, strongly disapproves that Harry is using the Prince's textbook, believing it is cheating and also dangerous. Why doesn't she report him?
- How could Salazar Slytherin's once-wealthy descendants have fallen to such a lowly social status? Why do they still consider themselves superior to other wizards?
- Considering her father's abusive treatment and her husband's cruel abandonment, why would Merope include "Tom" and "Marvolo" in her son's name?
- Even though Tom Riddle was duped by Merope, why would he abandon his innocent unborn child?
Greater Picture 
Several things in this little scene will prove important. This is the first time Slytherin's locket and the Peverell ring are seen. Both will be turned into Horcruxes by Voldemort, though we do not know yet what Horcruxes are. In the next book, readers learn why the ring's destruction is necessary, and how Dumbledore injured his hand. In the final book, it is revealed that the ring's stone is one of the three Deathly Hallows.
Harry chides Hermione for claiming that the Half-Blood Prince could be a girl. While he is quite right that a royal female is a princess and not a prince, in this particular instance Hermione was more accurate. The "prince" referred to here is not a royal title at all. Rather, it is someone's surname, and it could therefore have applied to either a male or female. Although Hermione is ultimately wrong that the book's previous owner was a female, nor does she realize yet it is someone's name, that surname did indeed belong to a woman, and she was related to the book's owner. Hermione demonstrates that her logic is more abstract and intuitive than Harry's typical linear reasoning. Rowling is dropping a subtle clue here that readers should look beyond the seemingly obvious.
The Half-Blood Prince's handwriting could possibly reveal his identity. On the notes that were pinned to Harry's Invisibility Cloak, the handwriting was described as "narrow [and] loopy." The note about Number 12, Grimmauld Place was a "narrow handwriting [that] was vaguely familiar." Both were written by Professor Dumbledore, as are a letter in "narrow, slanting handwriting" Harry received earlier in this book, as well as the note Harry just received, also in narrow, slanting handwriting. Harry recognizes the similar style; and we can suppose that Dumbledore deliberately made his writing more ornate and loopy when sending Harry the Cloak as a gift. However, we are not granted this clue regarding the Half-Blood Prince. In the previous book, Snape's grade on Harry's essay is described as a "large, spiky black 'D'," while the Half-Blood Prince's notations are noted as "small, cramped writing." Snape was the Half-Blood Prince, though the handwriting does not reflect this. Snape's writing may have changed over the years, or he adopted a different writing style to cram all his thoughts in the limited book margin space. A hint may have appeared in Harry Potter and the Order of the Phoenix when, using the Pensieve, Harry observes the young Snape as he is writing. Harry never notices any similarity, however, leaving us unable to make any connection via the writing style.