The mystery surrounding Angelina Jolie and Brad Pitt's choice of name for their newborn son Knox Leon has been solved - he is named after the actor's grandfather. Hal Knox Hillhouse, Pitt's maternal grandfather, died when the star was eleven years old. The name also means Knox shares something in common with his adoptive brothers Maddox and Pax - all their names end in x. Knox's sister Vivienne Marcheline is named after Jolie's late mother Marcheline Bertrand. The twins were born at a hospital in Nice, France, on Saturday .