The Half-Blood Prince's potions textbook, which contains notes and secrets from a mysterious former student, is a pivotal plot device. The subtitle "The Half-Blood Prince's Notes" could have been used, but instead, the book's title itself becomes a symbol of the story. The term "Half-Blood Prince" refers to Severus Snape, a character whose complex past and motivations are slowly revealed throughout the novel. The subtitle implicitly highlights Snape's significance and hints at his complicated relationships with Harry's parents, James and Lily Potter.