|Muggles' Guide to Harry Potter - Place|
|Permanent Residents||Slytherin students, The Bloody Baron|
|First Appearance||Harry Potter and the Philosopher's Stone|
Slytherin House is one of the four Houses in Hogwarts, and as such is both a location and a group of characters. The common room is located deep within Hogwarts, apparently in the Dungeons. Students are Sorted into Houses by the Sorting Hat upon their initial arrival at Hogwarts based on their character. While students remain at Hogwarts, they each represent their House; a tally of House points is kept, with good behaviour and good marks adding to that total, misbehaviour and bad marks subtracting from it. The House Cup is awarded each year to the House with the greatest tally of House points.
Founder: Salazar Slytherin
Coat of Arms: silver serpent, green
Ghost: The Bloody Baron
Characteristics: "These cunning folks use any means, To achieve their ends."
Location of common room: The Slytherin common room is a low-ceilinged, dungeon-like room with greenish lamps and chairs. It is located in the dungeons, behind an apparently blank stretch of wall. The author has commented that it is actually located under the lake.
While we see the inside of the Slytherin common room in Harry Potter and the Chamber of Secrets, Harry and Ron's sojourn there is necessarily brief. In Harry Potter and the Deathly Hallows, Harry claims that he was a member of Slytherin house to avoid identifying himself, and gives a description of the Common Room as proof. This description does not quite match what he saw in the earlier book, but it is sufficient to mislead his captors.
Residents of Slytherin house will be selected according to the biases of the founder of the house, Salazar Slytherin. As Slytherin is known to have departed the school in anger at the decision of the other School Founders to allow teaching of magic to those who had Muggle ancestry, it is not terribly surprising that those modern wizards who believe in the superiority of the Pure-blood wizards should end up Sorted into Slytherin house. This is also reflected in the common room password the one time we hear it used; at Christmas in Harry's second year, it is "Pureblood".
The number of students actually in each House is uncertain. We only ever hear of five boys and five girls in each year, but we have been told by the author in an interview that the average class size (all residents of one House for one year) is about thirty. Thus we must assume a population of 210 per house, fifteen boys and fifteen girls each year.
One gets the impression from things said in Harry Potter and the Goblet of Fire that House honours, notably the House cup and the Quidditch cup, are generally held by either Slytherin house or Gryffindor house, with Ravenclaw house occasionally winning an upset victory. It is to stymie Slytherin house's long run of House cup victories that Professor McGonagall chooses to relax the rule about First Year students and brooms for Harry.
One must wonder whether the placement of the Slytherin common room under the lake is an author's mistake. It is true that in the same book where we enter the Common Room, we also see the Chamber of Secrets, which also has a major connection to Salazar Slytherin, including a statue of him on the far wall. It is also true that Harry remarks on the water on the floor of the Chamber, which leads us to believe that the Chamber is located under the lake. It is certainly possible that the author had slightly confused the two locales, and having placed the Slytherin common room under the lake in an interview, then had to keep it there in later parts of the story.
As mentioned, one characteristic of Slytherin house is that "These cunning folks use any means, To achieve their ends." Because this is mentioned, in some form, whenever we hear the Sorting Hat singing about the Sorting, we can guess that this is the major criterion for selection into Slytherin House. Cunning does not seem to require intelligence, as we can see from some other characters from Slytherin House, notably Crabbe, Goyle, and later Amycus and Alecto Carrow. What does seem to be a common thread among those Slytherin students is a willingness to heighten their own status by putting down other students. This is particularly thoroughly displayed by Pansy Parkinson and Draco Malfoy, and by Lucius Malfoy, who we are unsurprised to find was a Prefect in Slytherin House some fifteen years previous to the opening of our story. The portrayal of characters from Slytherin seems to be designed to show how easily cunning can become bullying.
It is worth note that almost all boarding schools divide their students into multiple "houses", based either on the residence in which they are placed, some other characteristic, or simple random selection. This division fosters some internal competition, which can lead to improved performance in the school, and allows for practice with teams outside the school. There is possibly some benefit to grouping students by temperament; presumably, by grouping all the more Machiavellian students together ito Slytherin house, there will be less ground for complaint about the more egregious social climbing incidents, because in theory all students in the house will be trying to compete on those grounds.