First ever critical study of Tolkien`s little-known essay, which reveals how language invention shaped the creation of Middle-earth and beyond, to George R R Martin`s Game of Thrones.J.R.R. Tolkien`s linguistic invention was a fundamental part of his artistic output, to the extent that later on in life he attributed the existence of his mythology to the desire to give his languages a home and peoples to speak them. As Tolkien puts it in `A Secret Vice`, `the making of language and mythology are related functions``.In the 1930s, Tolkien composed and delivered two lectures, in which he explored these two key elements of his sub-creative methodology. The second of these, the seminal Andrew Lang Lecture for 1938-9, `On Fairy-Stories`, which he delivered at the University of St Andrews in Scotland, is well known. But many years before, in 1931, Tolkien gave a talk to a literary society entitled `A Hobby for the Home`, where he unveiled for the first time to a listening public the art that he had both himself encountered and been involved with since his earliest childhood: `the construction of imaginary languages in full or outline for amusement`.This talk would be edited by Christopher Tolkien for inclusion as `A Secret Vice` in The Monsters and the Critics and Other Essays and serves as the principal exposition of Tolkien`s art of inventing languages. This new critical edition, which includes previously unpublished notes and drafts by Tolkien connected with the essay, including his `Essay on Phonetic Symbolism`, goes some way towards re-opening the debate on the importance of linguistic invention in Tolkien`s mythology and the role of imaginary languages in fantasy literature.