J.R.R. Tolkien’s middle name between John and Reuel was Ronald.
His full name was John Ronald Reuel Tolkien. The initials in his familiar author name therefore represent John, Ronald, and Reuel. He was generally known as Ronald or Tolkien in different periods of his life, rather than using the complete name in ordinary conversation.
The name Reuel came from a family naming tradition and was also used by Tolkien’s father. Tolkien’s initials are so closely associated with his literary identity that many readers know him primarily as J.R.R. Tolkien, even though his full given names are recorded in biographies and reference works.