The prosecutor, Mr. Gilmer, questioned Mayella first (Chapter 18). He asked very few questions, wanting only to establish her testimony that Tom Robinson had raped her. In his examination, Atticus planed to ask her many questions, establishing her personality, family background, previous relationship with Tom, and other factors that would help the jury form a full picture of the situation. He finally manages to do this, but his plan initially does not go the way he expected.

When he begins to question Mayella, he continues with the humorous tone established by Gilmer, who had called him “big bad Mr. Finch.” He says “I won’t try to scare you….” As he does with all witnesses, Atticus is polite, calling her “Miss Mayella.” She quickly takes offense, thinking that he is “mocking” or “sassing” her, until the judge corrects her: “he’s trying to be polite.”

Scout, young as she is, notices that Mayella “took offense to routine courtesy.” This is unfathomable to someone who has been raised in her type of home. Scout wonders, “What on earth was her life like?” Atticus then proceeds with his plan, establishing how lonely and difficult was her life. At one point she challenges him again, having not been entirely convinced by the judge: “You makin’ fun o’me again, Mr. Finch?” This came in response to Atticus’ asking who her friends were. The implication is that she had none.

When Mayella is on the stand testifying, Atticus establishes reasonable doubt that Tom is guilty.

When Mayella is on the stand, Atticus treats her with respect.  She misinterprets this respect and believes that he is mocking her.  However, he is able to prove that she has a difficult life and was very lonely, and she was attacked by her father and not Tom Robinson.

Mayella looked from under lowered eyelids at Atticus, but she said to the judge: "Long's he keeps on callin' me ma'am an sayin' Miss Mayella. I don't hafta take his sass, I ain't called upon to take it." (CH 18)

By being polite to Mayella, Atticus establishes how young and unintelligent she really is.  She interprets common courtesy as sass, so she has not been the recipient of it before.  Atticus goes on to establish that Mayella was more afraid of her father than Tom Robinson, thus trying to establish reasonable doubt that Robinson attacked her.