There are several appendices located at the end of The Lord of the Rings which are typically found at the end of The Return of the King in traditional three volume publications.
J.R.R. Tolkien wrote them to detail the background of the Lord of the Rings, in part the Third Age before the Great Years. They provide family trees, annals of rulers, commentary on language, a chart of Elvish scripts, and a chronology of the Second and Third Ages.
There are six appendices: