The answer to this question can be found in chapter 12 of To Kill a Mockingbird. When Calpurnia takes Jem and Scout to the First Purchase Church, they are perplexed when the congregation rises to sing hymns. "How're we gonna sing it if there ain't no hymn books," Scout asks Calpurnia. The way that the congregation sings is different than their own church. Zeebo, the music superintendent, sings a hymn one line at a time from a hymnal, and the congregation repeats that line back to him. Scout is struck by the fact that they are all "on pitch" and moved by how the hymn ends in a "melancholy murmur." This, Calpurnia reveals, is called "linin.' " She says they use the technique because only about four people in the congregation, including her, can read. As for Calpurnia's changes in dialect, she tells the children that people would think she was "puttin' on airs" if she spoke otherwise at church. She believes that putting on airs would not change the speech of her fellow churchgoers (something both children seem to think would be appropriate) and in fact would offend them. So, she explains that she talks in ways that are appropriate to each of the two worlds she inhabits. This demonstrates the cultural chasm that exists along racial lines in Maycomb. The children are very close to Calpurnia, but they cannot fully know her. Scout asks if she can visit Calpurnia in her own world, demonstrating a desire to bridge that gap.