Which J.R.R. Tolkien essay appears in the collection Tree and Leaf?

On Fairy-Stories appears in J.R.R. Tolkien’s collection Tree and Leaf.

Tree and Leaf was first published in 1964 and brought together Tolkien’s essay On Fairy-Stories with his short story Leaf by Niggle. On Fairy-Stories developed ideas Tolkien had presented in a 1939 Andrew Lang lecture at the University of St Andrews.

The essay discusses the nature and purposes of fairy stories. Tolkien argues that such stories can offer recovery, escape, and consolation, and he also describes the process he calls “sub-creation,” by which a writer builds a secondary world. These ideas are frequently used to explain the artistic principles behind Middle-earth.

The collection’s title is sometimes mistaken for the title of a single essay or story. Leaf by Niggle is the fictional work in the book, while On Fairy-Stories is the critical essay. The Monsters and the Critics is a different Tolkien lecture and essay, published in another context.