The way that Calpurnia speaks at First Purchase is an example of code-switching. Code-switching is the practice of alternating between two or more languages or dialects in a single conversation or interaction. In the case of Calpurnia, she is switching between the dialect of her home community and the dialect of the African American community at First Purchase. This is a common practice among people who speak multiple dialects or languages and is often used to adapt to different social situations or to communicate more effectively with people who speak different dialects.