Pure-blood

"The pure-blood families are all interrelated. If you’re only going to let your sons and daughters marry pure-bloods your choice is very limited; there are hardly any of us left."

- Sirius Black

Pure-blood is the term wizards and witches who come from "purely" magical heritage call themselves. They claim to have no Muggles in their family trees. Because the only way for a family to retain their pure-blood status is to marry other pure-bloods, all pure-blood families were related to one another by the 1990s. Because of inbreeding among pure-bloods and some marrying Muggles and Muggle-borns, the number of families claiming to be pure-blood is declining.

At the very least, a witch or wizard must have all magical grandparents in order to be considered a pure-blood. However, the definitions of blood status were invented by those who are prejudiced against non-purebloods. They consider Muggle-borns to be as "bad" as Muggles, and therefore, it is likely that a wizard or witch must not have any Muggle-born grandparents either in order to be considered pure-blood.

Maintaining "Purity"
"The Death Eaters can't all be pure-blood, there aren't enough pure-blood wizards left. I expect most of them are half-bloods, pretending to be pure."

- Hermione Granger on the Death Eaters

Some pure-blood families can trace their pure-blood status through many generations of magical folk and deny ever having any Muggles within the family, such as the House of Black. However, the truth is that if they ever did exist in the past, true pure-blood wizards and witches do not exist today. They merely erase Squibs and Muggles from their family trees.

Those who consider blood purity very important also hide their Muggle ancestry, clinging to the magical heritage they do have. Many Death Eaters are believed to have done this, and their leader himself took on the name Lord Voldemort out of a desire not to keep the name of his "filthy Muggle father". It is likely that Voldemort told his followers he was a pure-blood, given the reaction of Death Eaters such as Bellatrix Lestrange to being informed he was in fact a half-blood.

Many pure-blood families, such as the Blacks and Gaunts, practice marrying cousins in order to maintain their pure-blood status, and disown any members who marry someone who is not a pure-blood. This practice has been known to cause problems as families become inbred; for example, the Gaunts showed signs of violent tendencies, mental instability, and even diminished magical capability. The number of pure-bloods is diminishing over time because of inbreeding.

Attitude
"I really don’t think they should let the other sort in, do you? They’re just not the same, they’ve never been brought up to know our ways. Some of them have never even heard of Hogwarts until they get the letter, imagine. I think they should keep it in the old wizarding families."

- Draco Malfoy

Many pure-bloods regard themselves as superior to those witches and wizards who are born to Muggles and half-bloods. They believe that Muggles are little more than animals, and that Muggle-borns, whom they derisively call "Mudbloods", are second-class citizens who are inferior and unworthy of being allowed to practice magic. Many also discriminate against half-breeds, werewolves, and magical creatures such as Centaurs.

Salazar Slytherin believed that only pure-bloods should be allowed to attend Hogwarts School of Witchcraft and Wizardry and educated in magic; this was the cause of his conflict with the other Founders and his leaving the school. However, he left behind the Chamber of Secrets in the hope that one day his heir would open the Chamber and release the basilisk within to target Muggle-born students. Tom Marvolo Riddle was this heir, and the basilisk petrified several Muggle-born students in 1992 and killed one student, Moaning Myrtle, in 1943, respectively.

Riddle, later known as Lord Voldemort, continued his persecution of those who were not pure-bloods (ironically being a half-blood himself) with his followers, the Death Eaters. They strived to place pure-bloods in control of the wizarding world and to rid it of Muggle-borns. When Voldemort succeeded in taking over the Ministry of Magic during the height of the Second Wizarding War, his followers created the Muggle-Born Registration Commission in order to weed Muggle-borns out of the population, and send them to Azkaban for allegedly stealing magic. Pure-bloods were also blatantly favoured within the Ministry and Hogwarts School of Witchcraft and Wizardry.

However, there are pure-bloods who do not have these beliefs, and instead treat all magical people and Muggles as equals. These pure-blood folk are often labeled "blood traitors" by the more fanatical pure-bloods.

Other

 * Based on the attitudes they express towards "blood traitors" and their affiliation with the prejudiced Draco Malfoy, Pansy Parkinson and Blaise Zabini are likely pure-bloods, or at least claim to be.

Author's Comments
On the subject of blood purity, J. K. Rowling has said the following:


 * The expressions ‘pure-blood’, ‘half-blood’ and ‘Muggle-born’ have been coined by people to whom these distinctions matter, and express their originators’ prejudice. As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as ‘bad’ as a Muggle. Therefore Harry would be considered only ‘half’ wizard, because of his mother’s grandparents. If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted ‘Aryan’ or ‘Jewish’ blood...the Nazis used precisely the same warped logic as the Death Eaters. A single Jewish grandparent 'polluted' the blood, according to their propaganda.

This statement may indicate that all of a wizard or witch's grandparents must have magical heritage — i.e. that he or she cannot have a single Muggle or Muggle-born grandparent — in order for him or her to be considered pure-blood. The detailed and lengthy family trees maintained by pure-blood families such as the House of Black suggests that more than two generations of magical heritage are required to be considered pure-blood. However, this may only be the standard applied by extremists.

Behind the scenes

 * On J. K. Rowling's draft list of students in Harry Potter's class, which is not considered canon due to several contradictions with the novels, the following students were mentioned as pure-bloods :
 * Lavender Brown
 * Stephen Cornfoot
 * Katherine MacDougal
 * Morag MacDougal