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 HousesEdit
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.