Probably the most obvious example of the Jim Crow laws in action comes at the beginning of the trial of Tom Robinson. The black men and women, who are patiently waiting "for the white people to go upstairs, began to come in." They were met by members of the Idler's Club.
"Whoa now, just a minute," said a club member, holding up his walking stick. "Just don't start up them there steps yet awhile." (Chapter 16)
It was an unwritten law that Negroes had to wait until all of the white spectators were seated before they could enter. Additionally, the blacks were not allowed to sit in the main section with the whites; they were segregated in the balcony. Later, Tom testifies that he had never entered the Ewell's property without permission. Scout reflects about Tom's respectability, and how
... a respectable Negro would never go into somebody's yard of his own volition. (Chapter 19)
When Calpurnia takes Jem and Scout to her church, she is met by Lula, who objects to Cal
"... bringin' white chillun to nigger church." (Chapter 12)
Lula knows that none of the black members of the First Purchase congregation would be allowed to enter any of Maycomb's white churches, so she does not see a reason for the white children to attend hers.
One excerpt in the book that deals with the Jim Crow laws is Jem's explanation of Dolphus Raymond's family in Chapter 16. Jem explains to Scout that Dolphus Raymond has biracial children with his African American mistress and that the children are sad. Jem says:
“They don’t belong anywhere. Colored folks won’t have 'em because they’re half white; white folks won’t have 'em cause they’re colored, so they’re just inbetweens, don’t belong anywhere."
Jem also tells Scout that Dolphus Raymond has sent some of his children up north, where they are more accepted. Jim Crow laws in the South meant that a person with any African American ancestry would be treated as a second-class citizen, and the rigid line between black and white left no room for other kinds of people.
In Chapter 19, Scout uses the logic of Jim Crow laws to explain why she feels that Tom Robinson is innocent. She says, "He seemed to be a respectable Negro, and a respectable Negro would never go up into somebody’s yard of his own volition." In other words, an African American person in Alabama had to know his or her place at that time; African American people would never venture into a white person's yard without being explicitly asked to do so because African Americans were well aware that they would be severely punished for doing so. There was a rigid color line, and Scout knows that Mayella Ewell must have invited Tom Robinson into her yard, as he wouldn't have gone without being asked.