In magical Britain, some families are considered to be Noble Houses which gives them some privileges. A few of them are called Noble and Most Ancient Houses which gives them supplementary privileges.
Most Ancient Houses[]
As of 1992, there a seven Noble and Most Ancient Houses. Among them are the House of Black (canon), the House of Greengrass, the House of Longbottom (canon), and the House of Malfoy. In Significant Digits, the House of Diggory is a Noble and Most Ancient House.
The last Most Ancient House to have become extinct is the House of Monroe whose members were killed by Voldemort and the Death Eaters during the Wizarding War.
If anyone ends a Most Ancient House, whoever avenges that blood is made Noble. Thus the Potter family and later the Granger family became Noble Houses when they avenged the Most Ancient House of Monroe by defeating Voldemort.
Only members of Most Ancient Houses have the right to use the charm of the Most Ancient Blade.