The Severians, also Severyans, Siverians, or Siverianians (Севяране; Севери; Северяне; Сiверяни) were a tribe or tribal confederation of early East Slavs occupying areas to the east of the middle Dnieper River and southeast of the Danube River. They are mentioned by the Bavarian Geographer (9th century), Emperor Constantine VII (956–959), the Khazar ruler Joseph (c. 955), and in the Primary Chronicle (1113). The etymology of the name "Severian" is uncertain. The name of the Severia region originated from the Slavic tribes. One theory proposes derivation from the Slavic word for "north" (sěver; men of the north), but the Severians never were the northernmost tribe of Slavs. Another theory proposes an Iranic derivation, from the name of the Sarmatian Seuer tribe (seu meaning "black"). Some scholars have argued that Jews called this tribe the Sawarta, based on the Kievan Letter (c. 930), written in Hebrew as SWRTH (read either as Sur'ata or Sever'ata), derived from Slavic sirota ("orphan"; in the letter, possibly meaning "convert"); or that the name "Severian" comes from the Magyar Savarti ("black"; possibly borrowed from Proto-Germanic swartaz). Based on the writings of the Bavarian Geographer, some scholars connect the ethnonym to the Zuierani, Zeriuani, or Sebbirozi (most probably the Sabirs). The Severians are believed to have continued the East Slavic tribal union along the middle Dnieper valley, after the political disappearance of the Antae and Dulebes, either independently or under Khazar policy. It is presumed they inhabited the lower Desna and upper Sejm and Sula rivers. They were thought to have been centered in Chernihiv ("black city"). However, as the Severians in the historical sources inhabited both the Dnieper valley and a part of the Danube valley, and as the Zeriuani realm was said to be so great that all Slavs traced their origins to it, Henryk Łowmiański believed that the Ruthenian Severians were the Slavic mother tribe. Professor Traian Stoianovich described the Severian Slavs as mix of Slavs and Slavicized formerly Turkic speaking Huns.