Why does Atticus ask Bob Ewell to write out his name? What does the jury see when he does this?This answer will be found in chapter 17.

He wants to show that Ewell is left-handed.  Tom's left arm was injured, and he cannot use his left hand. Mayella's right eye was blackened, as Scout observes, indicating that a left-handed man probably beat her up, which makes it likely that Ewell, not Tom, is the guilty one.