Harry learns that Grimmauld Place is Sirius Black's family home and the Order of the Phoenix's headquarters

PAGES: 79-80

As Sirius Black leads Harry Potter down to the basement kitchen of Grimmauld Place, he explains that this was his parents' house, which he has now offered to Dumbledore to serve as headquarters for the Order of the Phoenix. The conversation reveals Sirius's bitterness about his family and his isolated position, and it sets the stage for more revelations about the Order's activities.