As a result of the manner of its creation, the secondary world of Middle-earth is complicated. Its creator developed various elements of his fiction over the course of decades, making substantial changes including the abandonment of major themes, facts and entire tales, and undertook wholesale rewrites and revisions of otherwise complete narratives. The author's opinions on the relationships of his texts to each other often changed. In his letters, Tolkien comments upon the intertextual relationships of his works: