Tolkien conceived of The Lord of the Rings as a multiple volume with six sections he called "books" along with extensive appendices. The original publisher made the decision to split the work into three parts. It was also the publisher's decision to place the fifth and sixth books and the appendices into one volume under the title The Return of the King, in reference to Aragorn's assumption of the throne of Gondor. Tolkien indicated he would have preferred The War of the Ring as a title, as it gave away less of the story.
Before the decision to publish The Lord of the Rings in three volumes was made, Tolkien had hoped to publish the novel in one volume, or combined with The Silmarillion. At this stage he planned to title the individual books. The proposed title for Book I was The First Journey or The Ring Sets Out. Book II was titled The Journey of the Nine Companions or The Ring Goes South. The titles The Ring Sets Out and The Ring Goes South were used in the Millennium edition.
The Prologue is meant primarily to help people who have not read The Hobbit understand the events of that book, along with some other information that Tolkien felt was relevant to set the stage for the novel.