The entirety of To Kill A Mockingbird is narrated by the Adult Jean Louise, looking back on the events of a three year span of her childhood, beginning when she was six years old, and the events of a trial in which her lawyer father, Atticus, defended a black sharecropper named Tom Robinson against the charges of rape. In the very beginning of the novel, the voice of Adult Jean Louise sets the scene:
When enough years had gone by to enable us to look back on them, we sometimes discussed the events leading to his [Jem's] accident. I maintain that the Ewells started it all, but Jem, who was four years my senior, said it started long before that. He said it began the summer Dill came to us, when Dill first gave us the idea of making Boo Radley come out. [Chapter 1]
In this passage, the adult narrator is looking back on the inception of the events of the entire novel, which occur during Scout's childhood. The use of the adult voice of Jean Louise narrating events which are in her past gives her a clarity of expression and detail which would not have been possible with a child narrator.
A good example of the two voices working together can be found further along in Chapter 1, during the initial interaction between the children, Scout, Jem, and Dill:
When Dill reduced Dracula to dust, and Jem said the show sounded better than the book, I asked Dill where his father was: "You ain't said anything about him."
"I haven't got one."
"Is he dead?"
"Then if he's not dead you've got one, haven't you?"
Dill blushed and Jem told me to hush, a sure sign that Dill had been studied and found acceptable. Thereafter the summer passed in routine contentment. [Chapter 1]
In this passage, we hear the Adult Scout relating a conversation where the Child Scout has made a tactless inquiry about Dill's absentee father. It is strongly implied though never implicitly stated in the story that Dill is illegitimate, a fact which the Child Scout could not have known or understood; the Adult Scout relates the conversation, and shows, with an efficient use of details ('Dill blushed and Jem told me to hush'), that as an adult she understands that there was more to Dill's situation than the Child Scout could have understood. The dialogue is all the Child Scout's voice; the rendering and implicit analysis of the conversation is very adult.