Harry Discovers Regulus Black's Room and Identifies R.A.B.
Pages 185-187
As Harry and Hermione leave Sirius's room, Harry notices a sign on another door belonging to Regulus Black. Realizing that R.A.B. from the fake Horcrux is Regulus, Harry calls Hermione back, and they confirm that Regulus, a former Death Eater, must have discovered Lord Voldemort's secret. They decide to investigate the room for clues about the Horcrux.