What is the significance of Mayella's right eye being blackened and bruised in "To Kill a Mockingbird"?
It is significant, because it means she was most likely beaten by a left-handed man. Bruises often occur on the opposite side of the blows that create them. Atticus knows this, and during Robert Ewell's testimony it is revealed that he is left-handed. Atticus then asks Tom to stand, and everyone in the courtroom can see that it would have been nearly impossible for him to inflict the bruises, as his left arm was lost in an industrial accident years before. In fact, it was this misfortune that brought him into the Ewells' lives in the first place. Because he cannot do as much as he could before the accident, he does odd jobs for anyone who hires him. It is this which brings him into contact with Mayella.
If a white man had been on trial, this revelation would most likely have proved his innocence in the minds of the public. For Tom though, it simply shows that the racism of Maycomb runs so deep that they would convict an innocent man before they would challenge the testimony of a white man. So, even though Scout, Jem, and the audience realize that Tom could not have been the one to beat Mayella, the jury ignores the evidence, and follows instead their instincts of fear and oppression.
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.