There, they discover the significance of the letters R.A.B. In the previous book, Harry Potter and the Half-Blood Prince, Harry and Dumbledore had undergone trials to find a locket that Voldemort had made into a Horcrux, but at the end they found that the locket had been stolen, with a fake locket and note left behind, signed R.A.B. Now, they see that the initials belong to Sirius’s dead younger brother, Regulus Arcturus Black, who had been one of Voldemort’s followers. They remember that they have seen a locket in the house that is now gone.