The nobility of China was an important feature of the traditional social structure of Ancient China and Imperial China. While the concepts of hereditary sovereign and peerage titles and noble families were featured as early as the semi-mythical, early historical period, a settled system of nobility was established from the Zhou dynasty. In the subsequent millennia, this system was largely maintained in form, with some changes and additions, although the content constantly evolved. After the Song dynasty, most bureaucratic offices were filled through the imperial examination system, undermining the power of the hereditary aristocracy. Historians have noted the disappearance by 1000 AD of the powerful clans that had dominated China. The last, well-developed system of noble titles was establi