Mayella Ewell accused Tom Robinson of having beaten and raped her.  The evidence of this beating was her blackened right eye from the post-rape medical exam. However, Atticus Finch, Tom's lawyer, proves that Tom could not have been the one who bruised her right eye because an injury like that would require a left-handed fist.  Tom's left hand was injured when he was much younger and he is unable to use that hand, and is thus especially unable to make a fist to hit someone with it.  Bob Ewell, however, Mayella's violent-tempered father, is left-handed and, as Atticus Finch indirectly suggests during the trial, could have been the one who really hit Mayella.  This is possible because, according to Tom, Mayella came onto him and Bob Ewell saw her throw himsef at him through the window before he came in screaming at her and threatening to kill her.