Together the "Histories" and the "Annals" amounted to 30 books. These thirty books are referred to by Saint Jerome, and about half of them have survived. Although some scholars differ on how to assign the books to each work, traditionally fourteen are assigned to "Histories" and sixteen to the "Annals". Tacitus' friend Pliny referred to "your histories" when writing to him about his earlier work. Although Tacitus refers to part of his work as "my annals", the title of the work "Annals" used today was not assigned by Tacitus himself, but derives from its year-by-year structure.