What was the name of the fictional device Tolkien used to explain why Bilbo’s story appears within The Red Book?

The fictional device Tolkien used to explain the source of Bilbo’s story is The Red Book of Westmarch.

Within Tolkien’s fiction, The Red Book of Westmarch is a collection of writings associated first with Bilbo Baggins and later with Frodo and Samwise. It provides the imagined historical source for much of The Hobbit and The Lord of the Rings.

Bilbo records his adventures in There and Back Again, while Frodo writes an account of the War of the Ring. Sam completes and preserves the work, which later becomes associated with the Westmarch after the book is given to him. Tolkien’s framing makes the narratives resemble translated or transmitted historical records rather than ordinary modern fiction.

The Red Book should not be confused with The Book of Mazarbul, which is a separate record discovered in Moria. It is also different from The Book of Lost Tales, the title of an early collection of Tolkien’s mythological writings, later edited and published by Christopher Tolkien.