As can be deduced from its "Polybian" epithet, the most important extant literary source on the Roman army of this period are "The Histories" of the Greek historian Polybius, published in c. 160 BC. The surviving chapters cover the First and Second Punic Wars. Chapter VI contains a detailed analysis of the organisation and basic practices of the army. Polybius is generally seen by modern historians as a reliable and balanced source, but there are some inconsistencies and unclear points of detail in his account. These partly derive from his use of Greek terms to describe Roman military units and other terms. Moreover, the chronology of his account is uncertain. It has been suggested, from features such as joint consular armies, that he describes the army as it was c. 218 BC, at the start of the Second Punic War, considerably earlier than his time of writing (c. 160 BC). It is also possible that his account contains details from various historical periods. Polybius' source for Chapter VI remains uncertain. It has been suggested he was using an old army manual. The second most important literary source is "Ab Urbe condita", a massive history of Rome published in ca. AD 20, by the Augustan-era Roman historian Livy, whose surviving books XXI–XLV cover the years 218–168 BC. Although a narrative history lacking a specific analysis of the army as in Polybius, Livy's work contains much incidental information about the army and its tactics. Also useful are the monograph on the Jugurthine War by Sallust (published c. 90 BC) and the much later biographies of Roman leaders of the Republican period by Plutarch.