In Chapter 17, there is turning of events in the trial of Tom Robinson. Two points come to light. First, the bruises on Mayella's face were on the right side. The implication of this is that the person who beat Mayella was a left-handed person. Even Jem, a young boy, came to this conclusion. He said:
If her right eye was blacked and she was beaten mostly on the right side of the face, it would tend to show that a left-handed person did it. Sherlock Holmes and Jem Finch would agree.
During the trial, Atticus showed two very important points. First, he showed that Bob Ewell was left-handed. When Atticus asked him to write something, he used his left hand. Judge Taylor even remarked that Bob was left-handed.
The second point that came to light was that Tom Robinson was not left-handed. More specifically, Tom Robinson did not have a left hand. He was crippled. His left arm was badly hurt in an accident. The description of Tom's left arm is powerful:
He looked oddly off balance, but it was not from the way he was standing. 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 could see that it was no use to him.
These two point shows that Bob Ewell was the one who beat Mayella. The evidence demands this conclusion.