In Chapter 16 of To Kill a Mockingbird, Atticus reflects upon the events of the previous evening when a mob composed of the Old Sarum Bunch approached Atticus in order to get him to step aside from the jailhouse door and turn Tom Robinson over to them.
"You know, it's a funny thing about Braxton," said Atticus."He despises Negroes, won't have one near him."
As he makes this statement, Atticus is being served coffee by Calpurnia. After Calpurnia leaves the room, Alexandra chastises her brother, "Don't talk like that in front of Calpurnia." Further, she mentions that "they the African-Americans]talk among themselves."
Aunt Alexandra feels it is improper for her brother to discuss a white man and his attitudes before a servant and a black woman. She worries that Atticus breaks the social code that prohibits them from expressing any feelings about one another before someone of the other race.