What details in Mr. Ewell’s description of the alleged crime are most important to Atticus? How does Harper Lee help the reader know?

Harper Lee uses Scout, the narrator, to let us know which parts of Bob Ewell's testimony are the most important. For example, when Ewell testifies that he saw "that black n----r ruttin' on my Mayella," Scout describes the turmoil that broke out in the courtroom, which caused Judge Taylor to hammer his gavel for more than five minutes (231).

Scout reveals in great detail how Atticus called into question Ewell's testimony by demonstrating that he was ambidextrous, meaning he could use both his right and left hand equally. Lee stretches Atticus's cross-examination of Ewell over several pages, including Scout's fear that her father had "gone frog-sticking without a light" (237) by asking questions that he does not already know the answer to. Once it is established that Ewell is ambidextrous, Scout explains why this detail is important: Mayella's bruises were on the right side of her face, suggesting that she was punched by someone using their left hand. Later Atticus shows that Tom Robinson's left arm is permanently crippled. He could not have caused the bruise on Mayella's eye, and Scout shows the reader what the jury is unwilling to rule: that Bob Ewell beat his own daughter.