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 to Hannah Abbott after Harry Potter's ability to speak Parseltongue was revealed

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 more common to be hereditary. Nearly all known Parselmouths are descended from Salazar Slytherin with Harry Potter being the notable exception.

Parseltongue is, when spoken, a hissing sound, like how snakes hiss; as such, normal people cannot understand it. An exception to this was Albus Dumbledore, who though not a Parselmouth, was able to understand the language, presumably through study. Aside from serpent-based creatures, Parselmouths can communicate with each other with the language, as Harry understood Tom Riddle's commands to Salazar Slytherin's basilisk, and the Gaunt family 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 (i.e. a carving), as Harry required to make the carving of a small snake look like its moving through optical illusions to open the Chamber of Secrets, the Gaunt family seemed to be very proficient in the language, enough for Morfin to speak to Bob Ogden with it.

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. Although as Albus Dumbledore points out there are Parselmouths among the good as well, the most well known being Harry Potter.

Harry Potter as a Parselmouth
Lord Voldemort passed the ability to speak and understand Parseltongue on to Harry Potter when he attacked him in infancy in 1981, and inadvertantly and unknowingly made Harry into one of his Horcruxes. 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 boa constrictor while in the reptile exhibit. Harry inadvertently caused the glass of the snake's tank to vanish, enabling it to escape.

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. 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.

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, and was glad to lose it.

Known Parselmouths

 * Herpo the Foul
 * Marvolo Gaunt
 * Morfin Gaunt
 * Merope Gaunt
 * Salazar Slytherin
 * Tom Marvolo Riddle
 * Harry Potter (from 1981 to 1998)

Other Incidents of Parseltongue

 * Ginny Weasley was able to speak Parseltongue when she was possessed by Tom Riddle's Diary, which enabled her to open the Chamber of Secrets.
 * Ron Weasley was also able to imitate a fragment of Parseltongue he had heard Harry Potter use to open the Chamber during the Battle of Hogwarts, such as the command to "open" the doors, by making the same hissing sound Harry made before, though it took him several tries to work.
 * Albus Dumbledore understood, but did not speak, Parseltongue.
 * Bathilda Bagshot could speak it when Voldemort possesed her.
 * Hermione Granger once understood Parselmouth.

Etymology
J. K. Rowling has said 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.
 * In the Philosopher's Stone film, Harry converses with a Burmese python, rather than a Boa Constrictor.
 * In the films, Parseltongue is depicted as sounding like a sibilant-filled language with readily discernible phonemes. In the novels, it is only ever described as a low hissing noise.

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 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

Notes and references
Fourchelang Käärmesuu Sisseltong