During the trial, we learn that Tom's left arm is withered due to a childhood accident. This means that he simply could not have committed the crimes for which he's been charged. It would've been physically impossible for him to have attacked and raped Mayella.

In a fair trial, this information would've blown the prosecution's case right out of the water, but this is the Deep South in the 1930s; this is a place and a time in which racial prejudice is deeply ingrained in every aspect of life, including the criminal justice system. To the members of the jury, and to most white folk in town, Tom is guilty simply by virtue of being a black man charged with attacking a white woman. This is a legalized lynching, nothing more, and the jury understands what's expected of them. In spite of overwhelming evidence to the contrary, they convict Tom Robinson.

The fact that Tom has a lame left arm is found in Chapter 18. This fact is revealed in the way that Atticus gets Maybella to positively identify the defendant. She says she is certain it was Tom who grabbed, choked and raped her. Atticus asks Tom to stand:

...He rose to his feet and stood with his right hand on the back of his chair. 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 from the balcony I (Scout) could see that it was no use to him.

The implication, of course, is that Tom would have needed two fully functioning arms to commit the crimes he was accused of perpetrating.

In To Kill A Mockingbird, the reader learns a pertinent fact about Tom Robinson's physical condition. Tom Robinson had a crippled left arm. When Tom was a boy, his left arm had become crippled by a cotton gin. There is no way that he could have assaulted Mayella with his left arm. According to Mayella's brusies, which were on the right side of her face, she was attacked or beat by someone with a strong left arm. Tom Robinson could not have been her attacker. The fact that his left arm was crippled proves that Tom Robinson could not have beaten Mayella. 

Also, there were bruises around Mayella's neck. This indicates that someone with two good arms and hands tried to choke her. Again, Tom Robinson could not have brusied Mayella's neck. Also, when the reader considers that Tom Robinson was crippled in his left arm, this fact would provide evidence that he could not have raped Mayella. Mayella would have been able to free herself from Tom Robinson because he did not have two strong arms.