What legal points does Atticus use to defend Tom in To Kill a Mockingbird?
First, there was no witness to the crime aside from Mayella's own father, Bob Ewell. Therefore, it came down to the issue of Bob's and Mayella's statements against Tom Robinson's own side of the story. Secondly, no doctor ever tended to Mayella's injuries, so the severity of her injuries was only determined by Sheriff Tate, who was not a medical expert. The story takes place long before the days of the modern rape kit, in which the charge of rape could have been proven or disproven by the sampling of bodily fluids. Atticus is eventually able to establish that Mayella's right eye was blackened, and that such an injury was almost certainly inflicted by
"... someone who led almost exclusively with his left (hand)." (Chapter 20)
When Atticus asked Bob to sign his name, Ewell wrote with his left hand. Tom, as was seen by everyone in the courtroom, had a left arm
... that was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small, shriveled hand... (Chapter 18)
Although Atticus knew beforehand that no jury could be expected to take the word of a black man over the word of a white man, he must have thought he had proven his case. Tom's crippled arm prevented him from causing the injuries to Mayella's right eye, as well as the bruises around her neck. The left-handed Bob Ewell was the most likely suspect, since he must have been angry at seeing his daughter kissing and hugging a black man; and who, according to Tom's testimony, cried out
"... you goddam whore, I'll kill ya." (Chapter 19)