“That was how your House came to be ennobled, Mr. Potter,” injected the solemn voice of Professor McGonagall. “There is an ancient law that if anyone ends a Most Ancient House, whoever avenges that blood will be made Noble. To be sure, the House of Potter was already older than some lines called Ancient. But yours was titled a Noble House of Britain after the end of the war, in recognition that you had avenged the Most Ancient House of Monroe.”
Chapter 86