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The Lord of the Rings and The Hobbit are set during the Third Age of Middle-earth, while the First and Second Ages are explored in other Tolkien works such as The Silmarillion, Unfinished Tales, and The History of Middle-earth. Because Amazon only acquired the television rights to The Lord of the Rings and The Hobbit, the writers had to identify all of the references to the Second Age in those books and create a story that bridged those passages. These are primarily in the appendices of The Lord of the Rings.[17] Tolkien's estate were prepared to veto any changes from his established narrative,[37] including anything that contradicted what Tolkien wrote in other works.[17] The writers were free to add characters or details,[37] and worked with the estate and Tolkien lore experts to ensure these were "Tolkienian".[17] They referenced letters that Tolkien wrote about his mythology for additional context on the setting and characters.[17][53] Simon Tolkien, a novelist and the grandson of J. R. R. Tolkien, consulted on the series and helped develop its story and character arcs. He is credited as a "series consultant".[54]