common room is a long, low, dungeon-style room, under the Hogwarts Lake, furnished with green lamps and carved armchairs. The room is described in the second book as having a greenish glow. The Sorting Hat claims that blood purity is a factor in selecting Slytherins, although this is not mentioned until the fifth book. There is no reason to believe, however, that Muggle-born students are not sorted there, merely that pure-blooded students are more desirable to that house, as there are several examples of half-bloods in the house – such as Snape and Tom Riddle/Voldemort – and Harry himself was only excluded from the house at his own insistence. In Deathly Hallows, a group of Snatchers claim that "not many Mudbloods" are sorted into Slytherin. When believing Harry to be dead and thinking that he has final victory in his grasp, Voldemort proclaims his intention to abolish the other three houses and force all Hogwarts students into Slytherin. This design is foiled by his defeat and death, after which Slytherin becomes more diluted in its blood purity, no longer remaining the pure-blood bastion it once was. Subjects and teachers Being a school of magic, many subjects at Hogwarts differ from the studies of a typical school. Some subjects, such as History of Magic, derive from non-wizard – or muggle – subjects, but many others, such as charms and apparition classes, are unique to the wizarding world. There are twelve named teachers (referred to as Professors), each specialising in a single subject. All professors are overseen by a school head and deputy head. Transfiguration, Defence Against the Dark Arts, Charms, Potions, Astronomy, History of Magic, and Herbology are compulsory subjects for the first five years, as well as flying lessons. At the end of their second year, students are required to add at least two optional subjects to their syllabus for the start of the third year. The five choices are Arithmancy, Muggle Studies, Divination, Study of Ancient Runes and Care of Magical Creatures. According to J.K. Rowling, "very specialised subjects such as alchemy are sometimes offered in the final two years, if there is sufficient demand." At the end of their fifth year, students take the Ordinary Wizarding Level (O.W.L.) examinations for all subjects in which they are enrolled. Each examination consists of a written knowledge test and, where applicable, a practical demonstration of skills before a panel of proctors