House dynamics

House dynamics
The four houses are rather separate entities. Each has its own common room and dormitory, its own table in the Great Hall, and students mostly share lessons with classmates of their same house. There is no rule against students from different houses mingling, but in practice, a good majority of social interactions occur within the same house. Each house has one teacher who acts as its Head of House, along with its own ghost. Both Head of House and house ghost were previously pupils within that house.
A great deal of rivalry exists between the houses. This rivalry is demonstrated in the Quidditch matches and the annual school competition for the House Cup, which is determined by the number of points earned or lost by each member of the rival houses. These points are awarded or taken based on students' conduct throughout the school year. All members of the faculty have the authority to grant and deduct points as they see fit in each instance.[1] Some, like Snape, the former Potions master, tend to favour their own house, but that does not seem to be against the rules. He is despised by many students, other than those in Slytherin, where he is very popular. It is unknown how much favouritism the Hufflepuff and Ravenclaw Heads show, although it can be deduced that it is minimal or non-existent, as both Professor Filius Flitwick (Ravenclaw) and Professor Pomona Sprout (Hufflepuff) are kind, light-hearted people, who obviously want to win the House and Quidditch Cups but are not as vindictive as Snape in going about it. It is known that Professor Minerva McGonagall, the stern head of Gryffindor, does not show much favouritism towards her house. In fact, she deducted fifty points apiece and gave detention to three students in her house (Harry, Hermione, and Neville) when they were caught out of bed after hours in the first book, and supported Snape wholeheartedly in giving Harry detentions for the rest of the school year, even on overlapping Quidditch matches, after Harry used the Sectumsempra spell on Draco Malfoy in book six.
It has also been said that all four houses must band together and fight as one if Hogwarts is to stand. It must be remembered that all of the Houses have their virtues and flaws: none are inherently good or evil. Slytherins may have earned their reputation as evil because they show such contempt towards Gryffindor, the protagonist house. They also do not show much generosity towards other houses either, whether those in them are pure-blood or not.
House rivalry is most seen between Slytherin and Gryffindor ("Gryffindor and Slytherin students loathed each other on principle"[HP6]). This rivalry likely goes back to the days of Godric Gryffindor and Salazar Slytherin after they founded Hogwarts, because the Sorting Hat said that they were the best of friends before they founded the school.[HP5] While it is not clear if there is a similar rivalry between Ravenclaw and Hufflepuff, it seems very much unlikely due to the cheerful disposition of the Heads for said Houses, as opposed to the contempt of Professor Snape and the passion of Professor McGonagall (Rowena Ravenclaw and Helga Hufflepuff, however, once great friends, are documented as having had a severe falling out at some point by the Sorting Hat's song in Harry Potter and the Order of the Phoenix)[HP5].
Usually, though not always, the other two houses appear (in Harry's eyes) to support Gryffindor in its rivalry with Slytherin, which again is reminiscent of how Slytherin was in the end opposed by all three other founders. Dumbledore's Army is a representation of the unity between these three houses, as all of its members are Gryffindors, Ravenclaws, and Hufflepuffs. There are no Slytherin members. However, one exception to this occurred when Ravenclaw, Hufflepuff, and Slytherin all initially supported Hufflepuff's Cedric Diggory instead of Gryffindor's Harry Potter in the Triwizard Tournament.[HP4] Also in the first Quidditch match of book 6, it is said that many of the Hufflepuffs and Ravenclaws had "taken sides" in the Gryffindor vs Slytherin match, but does not specify which Houses took which side (due to the nature of previous books however, it is likely that they chose Gryffindor over Slytherin).
Before the Battle of Hogwarts, Hufflepuff and Ravenclaw join Gryffindor in defending Harry Potter against Slytherins willing to turn Harry in to Voldemort. After Slytherin Pansy Parkinson screams for someone to "grab Harry Potter", students from the other three houses rise in a "massive movement" and stand facing the Slytherins with their wands drawn. In addition, when the school is evacuated, seventh-year students are given the option of staying to fight; many Ravenclaws, even more Hufflepuffs, and nearly half of all Gryffindors remain, but all of the Slytherins leave; Voldemort later remarks that several of them have even joined their parents on his side. However, in the final battle, Slughorn and a number of unnamed Slytherins return with reinforcements.[2] Slughorn, whose loyalties are initially called into question by McGonagall, eventually duels Voldemort himself.[HP7]. When Harry visits the headmaster's office following the battle, Phineas Nigellus Black notes Slytherin's contribution. This seems to indicate that the actions of Snape, Slughorn, and his allies went a long way towards redeeming his former house's standing in the school's history.
Each of the four Hogwarts houses has its own Quidditch team. In the first book, Ravenclaw wins the Quidditch final. In the second book, the tournament is not finished due to the opening of the Chamber of Secrets. In the third, fifth, and sixth books, Gryffindor wins the Quidditch finals. Quidditch is not played in the fourth book because of the Triwizard Tournament. Quidditch is not covered in the seventh book because the protagonists spend the year away from Hogwarts.