What is an Epigraph? The quote at the top of a chapter.
In literature, an epigraph is a phrase, quotation, or poem that is set at the beginning of a document or component. The epigraph may serve as a preface, as a summary, as a counter-example, or to link the work to a wider literary canon, either to invite comparison or to enlist a conventional context.
The long quotation from Dante's Inferno that prefaces T. S. Eliot's The Love Song of J. Alfred Prufrock is part of a speech by one of the damned in Dante's Hell. Linking it to the monologue which forms Eliot's poem adds a comment and a dimension to Prufrock's confession. The epigraph to Eliot's Gerontion is a quotation from Shakespeare's Measure for Measure.
The epigraphs to the preamble of Georges Perec's Life: A User's Manual (La Vie mode d'emploi) and to the book as a whole warn the reader that tricks are going to be played and that all will not be what it seems.
The epigraph to E. L. Doctorow's Ragtime quotes Scott Joplin's instructions to those who play his music, "Do not play this piece fast. It is never right to play Ragtime fast." This stands in contrast to the accelerating pace of American society at the turn of the 20th century.
The epigraph to Dostoevsky's The Brothers Karamazov is John 12:24. "Verily, verily, I say unto you, except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit."
Some authors use fictional quotations that purport to be related to the fiction of the work itself. For example, Stephen King's Misery has epigraphs taken from the fictitious novels written by the protagonist; Jasper Fforde's The Eyre Affair has quotations from supposedly future works about the action of the story. Some science fiction authors (Isaac Asimov's Foundation Trilogy and Frank Herbert's Dune series are examples) are fond of using quotations from an imagined future history of the period of their story. This can be seen as a way of constructing authenticity for a work of the imagination.