“Remember it’s a sin to kill a mockingbird.” That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.

These lines from Chapter 10 are the source of the novel’s title and introduce one of the key metaphors of the book: the idea of “mockingbirds” as good, innocent people who are destroyed by evil. Boo Radley, for instance, is like a mockingbird—just as mockingbirds do not harm people but only “sing their hearts out for us,” Boo does not harm anyone; instead, he leaves Jem and Scout presents, covers Scout with a blanket during the fire, and eventually saves the children from Bob Ewell. Despite the pureness of his heart, however, Boo has been damaged by an abusive father. The connection between songbirds and innocents is made explicitly several times in the book: in Chapter 25, Mr. Underwood likens Tom Robinson’s death to “the senseless slaughter of songbirds by hunters and children”; in Chapter 30, Scout tells Atticus that hurting Boo Radley would be “sort of like shootin’ a mockingbird.” The moral imperative to protect the vulnerable governs Atticus’s decision to take Tom’s case, just as it leads Jem to protect the roly-poly bug from Scout’s hand.