Parseltongue

"Hannah, he’s a Parselmouth. Everyone knows that’s the mark of a Dark Wizard. Have you ever heard of a decent one who could talk to snakes? They called Slytherin himself Serpent-tongue."

- Ernie Macmillan

Parseltongue is the language of serpents (as well as other magical serpent-based creatures, like the Runespoor) and those who can converse with them. An individual who can speak Parseltongue is known as a Parselmouth. It is a very uncommon skill, and is typically hereditary. Nearly all known Parselmouths are descended from Salazar Slytherin with Harry Potter being a notable exception.

Characteristics
Parseltongue is, when spoken, a hissing sound, similar to that of a snake; as such, normal people cannot understand it (one known exception being Dumbledore). Apart from merely communicating with serpentine lifeforms, Parselmouths also seem able to influence the will of serpents to a certain extent. Aside from serpent-based creatures, Parselmouths can communicate with each other with the language, as Harry understood Tom Riddle's commands to the Serpent of Slytherin, and the House of Gaunt communicate with each other almost exclusively in Parseltongue. While inherited, Parseltongue usually requires the speaker to face a snake-based creature or object shaped like a snake (e.g. a carving); more proficient speakers may be able to speak it at will, as the Gaunt family was very proficient in the language - enough so for Morfin to speak to Bob Ogden with it, while Voldemort could speak to the Statue of Slytherin to release the basilisk.

Reputation
"The boy can talk to snakes, Dumbledore, and you still think he's trustworthy?"

- Cornelius Fudge and his prejudice against Parselmouths

The ability to actually speak Parseltongue — not just imitate it as Ron Weasley did — is considered an attribute of a Dark Wizard, which is partly due to the fact that both Salazar Slytherin and Lord Voldemort possessed this ability. Another notable Dark Wizard who was also a Parselmouth was Herpo the Foul. However, as Albus Dumbledore pointed out, there are Parselmouths among the good as well, the most well-known of which was Harry Potter. Paracelsus, a notable alchemist and a "medical genius", is credited with the discovery of Parseltongue in the sixteenth century, despite the existence of speakers like Herpo the Foul or Salazar Slytherin in the classical and medieval ages.

An unknown member of the Dark Force Defence League once stated when asked regarding the matter by Rita Skeeter - "Personally, I would be highly suspicious of anybody who could converse with snakes, as serpents are often used in the worst kinds of Dark Magic, and are historically associated with evildoers."

Harry Potter as a Parselmouth
"I spoke a different language? But I didn't realise it! How can I speak a language without knowing I can?"

- Harry Potter after he speaks Parseltongue.

Lord Voldemort passed the ability to speak and understand Parseltongue on to Harry Potter when he attacked him in infancy in 1981, inadvertently and unknowingly making him into a Horcrux. Harry first experienced conversation with snakes at age ten when he was taken to the zoo with his cousin Dudley Dursley and found himself communicating with a snake while in the reptile exhibit. Harry inadvertently caused the glass of the snake's tank to vanish, enabling it to escape. When the glass returned, he trapped his cousin Dudley inside the tank whilst laughing at him, and not knowing what to do about it.

Harry was not consciously aware of his ability to speak Parseltongue until 1992; he was pitted against Draco Malfoy in the Duelling Club and was able to communicate with the snake Draco conjured to attack him, telling it not to attack Justin Finch-Fletchley. This caused hysteria among his fellow students because graffiti on the walls of Hogwarts Castle foretold that the Heir of Slytherin would open the Chamber of Secrets, unleashing a monster that would attack the school's Muggle-born students. The Heir of Slytherin was in fact Voldemort, but Harry was able to gain access to the Chamber by speaking the password in Parseltongue, and subsequently killed the basilisk within. It was shortly after this incident that Harry learned how he acquired the ability.

When the wizarding world discovered Harry's ability to speak Parseltongue, courtesy of Rita Skeeter, people began to doubt his words, as Cornelius Fudge refused to believe Harry's claim of Voldemort's rebirth partially due to being prejudiced to Harry's ability to speak the language.

Harry also used his skill in Parseltongue to open Salazar Slytherin's Locket. When Harry, Ron Weasley, and Hermione Granger returned to Hogwarts to search for Voldemort's final Horcrux, Ron managed a weak imitation of Parseltongue to re-open the Chamber of Secrets so that he and Hermione could fetch basilisk fangs, which Hermione used to destroy Helga Hufflepuff's Cup.

After Lord Voldemort destroyed the fragment of his soul residing in Harry, Harry lost the ability to speak Parseltongue, about which he was glad and relieved.

Known Parselmouths

 * Herpo the Foul
 * Salazar Slytherin
 * Gormlaith Gaunt
 * Rionach Steward (possibly)
 * Corvinus Gaunt
 * Marvolo Gaunt
 * Morfin Gaunt
 * Merope Gaunt
 * Voldemort
 * Harry Potter (from 1981 to 1998)

Other incidents of Parseltongue

 * Ginny Weasley was able to speak Parseltongue while she was possessed by Tom Riddle's Diary, which enabled her to open the Chamber of Secrets.
 * Ron Weasley was also able to open the Chamber of Secrets during the Battle of Hogwarts by imitating a fragment of Parseltongue he had heard Harry Potter use to open Salazar Slytherin's Locket, though it took him several tries to make it work.
 * Albus Dumbledore could understand Parseltongue, but was unable to speak it.
 * Bathilda Bagshot's carcass spoke in Parseltongue while it was occupied by Nagini.

Etymology
J. K. Rowling has stated that she took the name Parselmouth from an "old word for someone who has a problem with the mouth, like a hare lip".

Behind the scenes

 * When Harry Potter and Hermione Granger went to Godric's Hollow in 1997, Nagini (inside the corpse of Bathilda Bagshot) tells Harry to "Come!" from the next room in Parseltongue. In reaction, Hermione jumps and clutches Harry's arm, and the two of them obey the command. It is unknown how Hermione seemed to understand this command, though it could be that she did not hear it well, and merely followed Harry. She could also have been frightened by the hissing sound she heard and then followed Harry.
 * In the Philosopher's Stone film, Harry converses with a Burmese python, rather than a Boa Constrictor.
 * Whenever it was spoken in the films, Parseltongue went untranslated except for one scene in Harry Potter and the Deathly Hallows: Part 2 when Voldemort was speaking to Nagini, where it was given subtitles. It was also heard in English in the first film when Harry was talking in the Zoo.
 * In the films, Parseltongue is depicted as sounding like a sibilant-filled language with readily discernible phonemes created by Dr. Francis Nolan (as listed in the credits for Harry Potter and the Deathly Hallows). In the novels, it is only ever described as a low hissing noise.
 * In the films, as Harry draws near a Horcrux he can hear, in addition to a whining noise, a faint voice which seems to speak in Parseltongue.
 * It is possible that Parseltongue comes in variants for different languages, rather than being understood as the language spoken by the Parselmouth. The boa constrictor that Harry Potter converses with at the Zoo uses the Portuguese word amigo rather than the English friend.
 * It is currently unknown whether there are any remaining Parselmouths in the Wizarding world, though it is possible that it could be a dead language like Latin.
 * Since Harry was a parselmouth because of the fragment of Voldemort's soul in him, it is possible that he could've commanded the Basilisk if he tried.
 * Although in the film version of the Chamber of Secrets, the version of Voldemort emitting from his diary claimed that the Basilisk would not listen to Harry's commands in Parseltongue as it would only obey Lord Voldemort. However, this was prior to the revelation of Harry being a Horcrux.

Appearances

 * Harry Potter and the Philosopher's Stone
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix (film) 
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2
 * LEGO Harry Potter: Years 1-4
 * LEGO Harry Potter: Years 5-7
 * Harry Potter for Kinect
 * Harry Potter: The Character Vault (mentioned only)

Notes and references
Fourchelang Serpentese Sisseltong Змеиный язык Wężousty pt-br:Ofidioglossia パーセルタング