In To Kill a Mockingbird, Sherriff Heck Tate is the first person called to the witness stand. The prosecutor asks Sherriff Tate to tell him about the crime. Heck Tate tells the court what Bob Ewell told him. Bob claims a black man came in and attacked Mayella and raped her. When Heck questions her she says it was Tom Robinson.
However, when Atticus questions him, we see the beginning of Atticus' defense for Tom. Heck Tate admits that Mayella had bruise marks all around her neck and was beaten badly on the right side of her face, indicating that the assailant would have had to have been left handed. Atticus is setting up reasonable doubt.
Heck Tate's testimony is one of the most important of the case. He is the Sherriff and people believe him, so when he says that the man must have been left handed, we trust his words. He is the first one to show us the doubt that we have all been feeling is in fact real. If only the rest of the town could have felt the same way.