Jem and Scout were practically brought up by the Finch family housekeeper, Calpurnia, in Harper Lee's novel, To Kill a Mockingbird. Cal taught Scout to write and she served as a surrogate mother and disciplinarian to both children. They were used to hearing Cal speak, or at least they thought so until they were given the chance to visit her church: the First Purchase African Methodist Episcopal Church "in the Quarters outside the southern town limits."
Jem and Scout soon notice that Calpurnia's speech patterns have changed drastically inside the church. They also discover that she is one of the only members of her church who can actually read. Naturally, Scout asked why in her typical, plain English.
"Cal," I asked, "why do you talk nigger-talk to the--to your folks when you know it's not right?"
"... It's right hard to say."
Cal went on to explain that she spoke differently at the Finch home and at her house because "it'd be out of place" otherwise, just as it would be if Jem and Scout spoke "colored folks' talk" at their own home. Cal added that
"... They'd (her friends) think I was puttin' on airs to beat Moses...
"It's not necessary to tell all you know. It's not ladylike--in the second place, folks don't like to have somebody around knowin' more than they do. It aggravates 'em..."
Cal's "double-talk" appealed to Scout, who asked to come visit her at her own home some time. "Any time you want to," Calpurnia replied.