In the trial scene in Part II of To Kill a Mockingbird, Bob Ewell's left-handedness is the smoking gun that implicates him in the beating of Mayella. Atticus gets Bob to write his name, and the jury notices that he's left-handed. Atticus says that since Mayella's bruises were on the right side of her face and that she was choked with two sets of good hands around her gullet, Bob Ewell is her attacker and not Tom.
Atticus' clever tactics should lead to a not guilty verdict from the jury, since Tom's left hand has been crushed in a machine. But, alas, racism makes the jury blind to the facts.