In Chapter 25 of Harper Lee's To Kill a Mockingbird, Scout reads an editorial written by Mr. B. B. Underwood that helps her better understand the injustice of Tom Robinson's trial.
Being young, at first, Scout has difficulty understanding Mr. Underwood's editorial. At first, Scout believes that Mr. Underwood is only arguing it is a "sin to kill cripples, be they standing, sitting, or escaping," a sin that he likens to "hunters and children" senselessly killing songbirds. Being young, Scout is puzzled as to why Mr. Underwood calls Robinson's death a "senseless killing." In Scout's view, Robinson had been given "due process of law" simply because he was given a trial. His trial was fair because her father had "fought for him all the way." She further views Robinson's trial as fair because he was "convicted by twelve good men and true"; therefore, Robinson was also killed for just causes. However, Mr. Underwood's editorial serves as a real eyeopener to the actual inherent unfairness of Robinson's trial.
After reading Mr. Underwood's editorial, Scout soon comes to understand that, due to racial prejudices, Robinson had been believed to be guilty the moment Mayella Ewell accused him. Since he was believed to be guilty, unlike others tried in the US, Robinson did not benefit from the presumption of innocence, an ancient legal tenet that presumes one charged with a crime is innocent until proven guilty. Since Robinson did not benefit from the presumption of innocence due to racial prejudices, his trial can automatically be seen as unfair. Scout explains her revelation about the unfairness of Robinson's trial in the following comment:
Then Mr. Underwood's meaning became clear: Atticus had used every tool available to free men to save Tom Robinson, but in the secret courts of men's hearts Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed. (Ch. 25)
Hence, though a legal trial can be considered "fair" in the sense that it is overseen by a judge, includes a defense attorney, and involves the unanimous judgement of a 12-man jury, a legal trial contrasts significantly with the "courts of men's hearts." In the "courts of men's hearts," people are judged to be guilty based solely on racial prejudice, regardless of contrary evidence. Through reading Mr. Underwood's editorial, Scout was finally able to see that Robinson's trial, though held in a courtroom, was really a trial held in "men's hearts" and therefore unfair.