Since its emergence as a centralized polity Silla society had been characterized by its strict aristocratic makeup. Silla had two royal classes: "sacred bone" ("seonggol", 성골, 聖骨) and "true bone" ("jingol", 진골, 眞骨). Up until the reign of King Muyeol this aristocracy had been divided into "sacred bone" and "true bone" aristocrats, with the former differentiated by their eligibility to attain the kingship. This duality had ended when Queen Jindeok, the last ruler from the "sacred bone" class, died in 654. The numbers of "sacred bone" aristocrats had been decreasing for generations, as the title was only conferred to those whose parents were both "sacred bones", whereas children of a "sacred" and a "true bone" parent were considered as "true bones". There were also many ways for a "sacred bone" to be demoted to a "true bone", thus making the entire system even more likely to collapse eventually.