Infamous publication from the 1930s which listed the supposed ‘Sacred Twenty-Eight’ families who have maintained a pure bloodline through history. The stated goal of the directory was to help pure-blood families keep their bloodlines pure by knowing with which families to intermarry.
Some of these families on the list, notably the Weasley family, protested being included as they were proud of being connected to many famous Muggles. Others families protested because they were not included in the list. The author of the directory remained anonymous, although many believe it to have been Cantankerus Nott.
Rowling discusses the subject of blood status is several Pottermore essays, notably 'Blood Status' and 'The Malfoy Family.'