In To Kill a Mockingbird, Mr. Bob Ewell, the town drunk, has accused Tom Robinson of raping his daughter, Mayella Ewell, and beating her in the process as well. Mr. Tate claims that he found Mayella with her right eye blackened, bruised arms, and finger marks all around her neck indicating that she had been choked.
During the court proceedings, Atticus tricks Mr. Ewell into demonstrating that he is left-handed and not ambidextrous; the injuries on Mayella's right side could only correlate with a left-handed person. Atticus' suspicions that it was Mr. Ewell who beat his own daughter are confirmed when Tom's deformity is revealed to the court:
His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and... it was of no use to him.
This is significant to his trial because it would have been physically impossible for Tom to do the things accused of him when his arm was crippled in such a manner.
We learn the truth of what happened from Tom's testimony: Mayella, "the loneliest person in the world," made sexual advances towards Tom. Tom was in the habit of regularly helping Mayella with chores. After luring him into the cabin with a request to help her with something, Mayella grabbed Tom around the waist and started kissing him. Tom tried to escape, but Mr. Ewell spotted the two and beat the girl himself after calling her a "whore."
In Chapter 18, Atticus has Tom Robinson stand up in order to have Mayella look at him and identify him as the man she accuses of attacking her. Significantly, "His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I [Scout)] could see that it was of no use to him." He had caught his left arm in a cotton gin when he was a boy. It appeared that Tom could not have inflicted Mayella's facial injuries with his right hand and it seemed obvious that she must have been beaten by someone else, presumably her own father. It also seemed impossible that Tom could have choked her or raped her, as she claimed, with only one good arm and hand.