Sorting

Sorting
In the early day of Hogwarts, the four founders handpicked students for their Houses. When the founders worried how students would be selected after their deaths, Godric Gryffindor took his hat off and they each added knowledge in it, allowing the Sorting Hat to choose the students. Now, at the beginning of each school year, the magical Sorting Hat is placed on each new student's head, during the Sorting Ceremony. The Sorting Hat announces the house the student is bound to join. Before beginning the Sorting ceremony, it sings a short song about the founding of the school and the four houses; the Hat's songs are different every year, and the Hat is said to take all year composing the next one (according to Ron Weasley, at least). The songs tend to give advice in troubled times.[HP5]
The Hat sorts by judging each student's qualities and placing them in the most appropriate house. The student's own choices may affect the decision: the clearest example is the Hat telling Harry Potter that he would do well in Slytherin in the first book, but ultimately selecting Gryffindor after Harry asks it not to put him in Slytherin. Another example is in the fifth book when Hermione Granger states that the Sorting Hat considered sorting her into Ravenclaw, but after she clearly stated in a previous chapter that she wanted to be in Gryffindor, her choice probably ultimately overrides the Hat, resulting in her placement into Gryffindor. Albus Dumbledore later explains to Harry that this is how it should be, when one considers how a person's choices define them more than their abilities. It is in fact likely that the Hat selects houses according to the predetermined decisions of an informed student, and only genuinely selects those who have no preconceptions or expectations. According to Horace Slughorn, house affiliations tend to run in families, and it is notable that Ron could not consider being in any house other than that which every other member of his family had been in; however, when Harry mentions this, Hermione points out that Parvati Patil and Padma Patil, who are identical twins, are sorted differently, into Gryffindor and Ravenclaw, respectively. Every member of the Black family had been sorted into Slytherin, with Sirius Black being the sole exception, which was remarked upon by Slughorn, its Head. On the other hand, many House members appear to share common traits: it is unknown if these are present before Sorting and used as criteria, or if they are inculcated into students through their presence in the House.
The Hat does not just consider the student's most obvious strengths; it also sees a student's potential qualities. "You could be great you know, it's all here in your head, and Slytherin will help you on the way to greatness", it says to Harry when it suggests Slytherin as the best house for his future. Similarly, Hermione and Neville Longbottom do not immediately jump out as prime examples of Gryffindor courage in the beginning of the series, but they have both gained a tremendous amount of courage as they have grown. Interestingly, in the final novel of the series, Dumbledore notes that Severus Snape is braver than most men are, and that perhaps Hogwarts sorts too early. Another point of interest is that Peter Pettigrew, who is frequently remarked as being cowardly and following others rather than acting on his own, was sorted into Gryffindor, where bravery is the main attribute of its students.