Which Tolkien essay contains his influential discussion of fairy-stories and “eucatastrophe”?

J.R.R. Tolkien’s essay On Fairy-Stories contains his influential discussion of fairy-stories and “eucatastrophe.” The essay grew from a lecture Tolkien delivered at the University of St Andrews in 1938 and was published in revised form in 1947.

Tolkien uses the term eucatastrophe for the sudden joyous turn at the end of a story. He presents it as the opposite of catastrophe, not because danger has never existed, but because the narrative’s final reversal brings unexpected hope. He also discusses recovery, escape, and consolation as important effects of fairy-stories.

The essay is often read alongside Tolkien’s fiction because it explains ideas visible in The Hobbit and The Lord of the Rings. It is not the same work as Beowulf: The Monsters and the Critics, Tolkien’s major 1936 lecture on the Old English poem. “Leaf by Niggle” is a separate short story with its own allegorical design.