Why could Tom not have choked Mayella or given her a black right eye in To Kill a Mockingbird?
During his questioning of Sheriff Heck Tate, Atticus establishes that Mayella Ewell has been beaten on the right side of her face, and that there were finger marks and bruises that went all around her neck. While Mayella is on the stand, Atticus has Tom Robinson stand so she can identify him. Scout immediately saw that Tom's
... left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small, shriveled hand...
While Bob is testifying, Atticus asks if he can write. Bob replies that he can, and then signs his name--with his left hand. The importance of the right and left hands is not immediately apparent to Scout, but Atticus explains in his summation that Mayella's injuries could only have been committed by a man with two good hands, and who led with his left, since Mayella's blackened eye was her right eye.
"... Mayella Ewell was beaten savagely by someone who led almost exclusively with his left... (Bob Ewell) swore out a warrant, no doubt signing it with his left hand, and Tom Robinson now sits before you, having taken the oath with the only good hand he possesses--his right hand.
Tom could not have choked Mayella because the bruises on her neck indicated that two hands were used, and Tom's left hand was "shriveled" and "dead." Mayella must have been punched in the eye by a left hand--Bob is left-handed--and certainly not by Tom's.