At least some content in this article is derived from information featured in Harry Potter: Hogwarts Mystery. Spoilers will be present within the article.
The Pure-Blood Directory is a directory created in the 1930s that was published anonymously in Great Britain listing twenty-eight families judged to be pure-blood by the anonymous author, who is widely believed to be Cantankerus Nott. He dubbed these families the Sacred Twenty-Eight. A number of families appearing on the list deplored their inclusion, with many, including the Weasleys, proudly noting that they had Muggle ancestors. Others felt there was a certain prestige in being on this list, and adherents to the pure-blood supremacy doctrine made quite a big deal out of their inclusion on this list of "pedigree".
The directory seems to have certain levels of bias and information inaccuracy, as the Potter family was excluded due to Henry Potter's outspoken pro-Muggle ideals and possible Muggle origins of their name, and the Ollivander family was included despite Garrick Ollivander's mother, a Muggle-born, was married into the family at the time of the directory's publishing.