In Chapter 17, Atticus questions Mr. Ewell and Mr. Tate. Atticus gets them both to confirm Mayella’s injuries. She was bruised on the right side of her face and all around her neck. Atticus establishes these facts to show that it is more likely that a left-handed person committed the crime. This is consistent with the bruises on Mayella’s right side of her face.
In Chapter 18, Atticus questions Mayella and she contradicts herself a few times. But she sticks to her story and also confirms the injuries Mr. Ewell and Tate agreed with. Finally, Atticus asks her to identify the man who raped and beat her and she identifies Tom. Tom stands up to reveal that his left arm is twelve inches shorter than his right and is basically useless. This casts huge doubt on Tom’s guilt because of the likelihood that Mayella’s attacker was left-handed. Also, the bruising “all around” her neck would most likely come from someone with two good hands.