In To Kill a Mockingbird by Harper Lee, why does Atticus point out that Tom's record has one blemish: he served 30 days in jail?
The climax of Harper Lee's To Kill a Mockingbird is the trial of a black man, Tom Robinson, who has been accused if raping a white woman--a certain death sentence in many Southern towns. In chapter nineteen of the novel, Atticus Finch begins questioning Tom, his client, on the stand.
The first thing he does is address Tom's previous criminal record--thirty days in jail for disorderly conduct.
“It must have been disorderly,” said Atticus. “What did it consist of?”
This simple question allows Tom to tell his story before the prosecuting attorney can use it against him. Atticus proceeds to prompt Tom to reveal the details of his previous incarceration.
“Got in a fight with another man, he tried to cut me.”
“Did he succeed?”
“Yes suh, a little, not enough to hurt. You see, I—” Tom moved his left shoulder.
“Yes,” said Atticus. “You were both convicted?”
“Yes suh, I had to serve ‘cause I couldn’t pay the fine. Other fellow paid his’n.”
His "crime" is such a harmless and understandable thing, and it is never mentioned again in the trial.
Dill asks essentially the same question as you, and for once Jem has the correct answer. Scout says it this way:
Dill leaned across me and asked Jem what Atticus was doing. Jem said Atticus was showing the jury that Tom had nothing to hide.
Atticus knows Tom's best defense is the truth, and this admission of a previous incarceration is the first step in that defense.
In chapter 19, Tom Robinson takes the witness stand, and Atticus begins by mentioning the one blemish on his client's record. Atticus tells the jury that Tom once served thirty days in prison after being charged with disorderly conduct. Tom admits to Atticus and the jury that he got into a fight with a man who ended up cutting him with a knife. Tom Robinson testifies that he served thirty days because he could not afford the fine. Scout then mentions that Dill leans over and asks Jem why Atticus is bringing up Tom's past transgression, and Jem tells Dill that Atticus is showing the jury that Tom has nothing to hide. Essentially, Atticus is attempting to portray Tom as an honest man, who has no intentions of lying to the jury while on the witness stand. Despite Atticus's best efforts, Tom is wrongly convicted of assaulting and raping Mayella and becomes a victim of racial injustice.