A callback, in terms of comedy, is a joke that refers to one previously told in the set. It is also known as an internal allusion, a literary device that helps give structure to the piece of writing. Callbacks are usually saved for later, towards the end of a set, the result is usually a much bigger laugh the second time as the audience is much more surprised the longer it's held back. The main principle behind the callback is to make the audience feel a sense of familiarity with the subject material, as well as with the comedian. It helps to create audience rapport. When the second joke is told, it induces a feeling similar to that of being told a personal or in-joke.