During the trial, Atticus asks Mayella, "He blacked your left eye with his right fist?" Yet, earlier in the book, readers are told that her right...

For the trial of Tom Robinson, there is no physical evidence to prove that he raped Mayella Ewell. There are also no other witnesses to the incident who could prove or disprove what actually happened that night. The whole trial is based on the Ewells' word against Tom's. Atticus's job is to make the jury doubt the Ewells and their story. He knows that the Ewells are lying; and with all liars, they tend to forget one lie to the next. So, if Atticus can trip Bob and Mayella up on the witness stand, then he will show that they don't even know the facts to their own lies, discredit their testimonies, and prove Tom is innocent.

Atticus first asks Sheriff Tate to verify which eye was blackened on Mayella's face in chapter 17. Tate says it was her right eye. Bob Ewell says during his testimony, "I hold with everything Tate said" (176), which is to say that both men maintain that Mayella's right eye was black. Once this is established twice, if Mayella says that her left eye was blackened, then Atticus can show that someone is lying. Also, when Atticus asks her, "He blacked your left eye with his right fist?" (186) he's trying to get her to admit that her left eye was black after two men said her right eye was black. This would show that she is a liar. Not only that, Tom is standing up before her--at Atticus's request--so that she and everyone in the courtroom can see that Tom could not have hit her on her right side because his left arm is handicapped.

Every question Atticus asks Mayella is to get her to either tell the truth, or through her verbal staggering, to show that she is a liar. She says one minute that she wasn't hit, then she says she was. She says he choked her, but she can't remember how. And just when it is so obvious that she is lying, she says, "That ni**** yonder took advantage of me an' if you fine fancy gentlemen don't wanta do nothin' about it then you're all yellow stinkin' cowards" (188). Since she can't answer the questions with confidence, she resorts to calling the jury cowards and saying nothing further. Atticus had done his job. It's too bad the jury sided with liars rather than with the innocent.