J.R.R. Tolkien’s full first name sequence was John Ronald Reuel. He was born John Ronald Reuel Tolkien, and the initials J.R.R. became the form by which he was internationally known.
The name Reuel came from a family tradition. Tolkien’s father was Arthur Reuel Tolkien, so Reuel was used as a middle name in the family. Tolkien’s first name, John, was connected with his paternal grandfather, while Ronald was the name generally used during his childhood and early adult life.
The initials can cause confusion because Tolkien is often informally called “Ronald Tolkien,” especially in biographical accounts of his youth. His published author name, however, was J.R.R. Tolkien. He later signed some letters and personal documents with variations of his name, but Reuel remained part of his official full name.