the eastern and western Roman Emperors, Basil I and Louis the German, quarreled over control of Bari, which had been besieged by Arabs. The Byzantine Emperor sent an angry letter to his western counterpart, reprimanding him for usurping the title of emperor. He argued that the Frankish rulers are simple reges, while the imperial title properly applied only to the overlord of the Romans, that is, to Basil himself. He also pointed out that each nation has its own title for the supreme ruler: for instance, the title of chaganus is used by the overlords of the Avars (Avari), Khazars (Gazari), and "Northmen" (Nortmanni). To that, Louis replied that he was aware only of the Avar khagans, and had never heard of the khagans of the Khazars and Normans. The content of Basil's letter, now lost, is reconstructed from Louis's reply, quoted in full in the Chronicon Salernitanum ("Salerno Chronicle]"). According to Dolger, it indicates that at least one group of Scandinavians had a ruler who called himself "khagan", but Ostrowski (2018) countered: 'The letter of Louis II to Basil I states specifically that the Northmen do not have a khagan. From that, the non-extant letter of Basil I has been thought to have stated that the Northmen had a khagan, but we do not know that. (...) Besides, even if Basil's letter did assert that the ruler of the Northmen was called a khagan, that testimony is negated by the statement of Louis II that their ruler is not called a khagan.' Arabic-Persian sources Ahmad ibn Rustah, a 10th-century Persian Muslim geographer, wrote that the Rus' khagan ("khāqān rus") lived on an island in a lake. Constantin Zuckerman comments that Ibn Rustah, using the text of the Anonymous Note from the 870s, attempted to accurately convey the titles of all rulers described by its author, which makes his evidence all the more invaluable. Ibn Rustah mentions only two khagans in his treatise—those of Khazaria and Rus. Hudud al-'Alam, an anonymous geography text written in Persian during the late 10th century ( 982–983), refers to the Rus' king as "Khāqān-i Rus". The unknown author of Hudud al-Alam relied on several 9th-century and 10th-century sources. Abu Said Gardizi, an 11th-century Persian Muslim geographer, mentioned "khāqān-i rus" in his work Zayn al-Akhbār. Ibn Rustah, the Hudud al-Alam and Gardizi all copied their information from the same late 9th-century source. Zuckerman (2000) argued