In the beginning of Chapter 10 Miss Maudie says it’s a sin to kill a mockingbird.
When his children get guns for Christmas, Atticus tells them that they can shoot blue jays but not mockingbirds. Scout asks Miss Maudie, because she has never heard her father say anything is a sin, and she explains why.
“Your father’s right,” she said. “Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.” (Ch. 10)
Different book editions have different page numbers. In the mass market paperback edition you will find this on page 117. It occurs near the beginning of Chapter 10, so as long as you can find Chapter 10, you should be able to find the quote pretty easily.
This phrase becomes a metaphor for the victimization of the innocent. For example, Mr. Underwood compares Tom Robinson’s death to “the senseless slaughter of songbirds by hunters and children” in an editorial. He does not believe that a cripple should be shot (Ch. 25).
Tom Robinson is a symbolic mockingbird because he never did anything wrong. He was targeted for being helpful to Mayella Ewell, who was a white woman. She was young and attracted to him, and this was something her father and the town of Maycomb could not accept.
Another symbolic mockingbird is Boo Radley. When Boo kills Bob Ewell after he attacks Scout and Jem, Atticus and Sheriff Heck Tate decide to pretend that Bob Ewell fell on his knife to protect Boo Radley from the publicity. He is shy and reclusive and would not like the attention that would come from everyone in town knowing what he did. When Atticus asks Scout if she can understand why they are keeping Boo’s heroism a secret, she explains that she does.
“Yes sir, I understand,” I reassured him. “Mr. Tate was right.”
Atticus disengaged himself and looked at me. “What do you mean?”
“Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?” (Ch. 30)
Scout compares Boo to a mockingbird because he has never done anything but be considerate and look out for the children, but he is largely misunderstood by the community. She understands the need to protect him. He has never done anything wrong, and has only tried to help the children. To expose him would be disastrous for him.
The book’s title comes from the understanding that Scout develops as she grows up. In the beginning of the book, she does not appreciate either Tom or Boo’s plight. However, over the course of the novel she grows up and comes to see why her father had to defend Tom Robinson and why Boo Radley watches them from a distance. She has learned how to walk around inside other people’s skin.