The middle initial R in J.R.R. Tolkien stands for Reuel.
His full name was John Ronald Reuel Tolkien. He was commonly known as Ronald or, in some family contexts, Ronald Tolkien rather than John. The name Reuel came from his father’s family and was used as part of Tolkien’s formal name.
Tolkien’s initials are especially familiar because his books were published under the abbreviated form J.R.R. Tolkien. The repeated initials distinguish him from other writers named Tolkien and became closely associated with The Hobbit and The Lord of the Rings.
The name should not be confused with Tolkien’s first given name, John, or with Ronald, the name by which he was widely known. His son Christopher Tolkien also became a major figure in preserving and editing his father’s writings, but he had a different full name: Christopher John Reuel Tolkien.