Founded by Salazar Slytherin, the house is composed mostly of pure-blood students, due to Salazar's mistrust of Muggle-born witches and wizards. The house has a negative reputation, with many claiming that it is the source of most Dark Wizards in Britain.
Slytherins tend to be ambitious, shrewd, cunning, and achievement-oriented. They also have highly developed senses of self-preservation. This means that Slytherins tend to hesitate before acting, so as to weigh all possible outcomes of a decision.
The Sorting Hat claims that blood purity is a factor in selecting Slytherins. This suggests that it bases its decisions more upon the views of its members than on any accurate test of a student's lineage. Muggle-born Slytherins exist, but are very rare. There have also been definite examples of half-bloods sorted into the house.
"There's not a single witch or wizard who went bad who wasn't in Slytherin."
Head of House: Severus Snape