Who are the ‘Sacred Twenty-Eight’?

In the 1930s or 1940s, an anonymous publication concerned with preserving the purity of bloodlines within the magical population was compiled, giving a compendium of the truly pure-blood families.

[kshmatafulzele]

They still call him He Who Must Not Be Named

[BennyKman]

I think it's 24 or less now with the deaths of crouch, Gaunts, and the last black becoming a Malfoy. Not to mention after the battle of Hogwarts. Does anyone know the exact number?

[aaravd]

Most of the sacred 28 bloodlines ended lol they need to make a new sacred 28

[bladencarroll]

They named only 27 pure blood family’s and there are 28 of the sacred “28”