on CMOS Shop Talk:
An epigraph is a brief quotation placed at the beginning of a book or at the head of a chapter, article, story, or other work. Most epigraphs are ornamental, helping to set the tone or mood of a work but going unmentioned in the text. They can be thought-provoking and profound or funny and frivolous. They don’t even have to be real.
If you’re planning on adding an epigraph to your novel or story, you will want to consider not only what to choose as your source but also whether you will need permission to quote from it. Then you or your publisher can decide where your epigraph will go and how to format it.
To get a better idea of how epigraphs work, let’s start with placement and format.