According to the Samguk Sagi, the principal source for events of this period, Yuri centralized rule over the aristocracy by turning the six tribes into six official administrative divisions of Silla. He is said to have granted surnames to each of the clans: Yi, Choe, Son, Jeong, Bae, and Seol. He is also said to have created 17 bureaucratic rank levels. However, modern scholars doubt that these occurred so early in Silla's development.