Suzerainty () is the rights and obligations of a person, state or other polity who controls the foreign policy and relations of a tributary state, while allowing the tributary state to have internal autonomy. While the subordinate party is called a vassal, vassal state or tributary state, the dominant party is called a suzerain. While the rights and obligations of a vassal are called vassalage, the rights and obligations of a suzerain are called suzerainty. Suzerainty differs from sovereignty in that the dominant power allows tributary states to be technically independent, but enjoy only limited self-rule. Although the situation has existed in a number of historical empires, it is considered difficult to reconcile with 20th- or 21st-century concepts of international law, in which sovereignty is a binary concept, which either exists or does not. While a sovereign state can agree by treaty to become a protectorate of a stronger power, modern international law does not recognise any way of making this relationship compulsory on the weaker power. Suzerainty is a practical, de facto situation, rather than a legal, de jure one. Imperial China The tributary system of China or Cefeng system was a network of loose international relations focused on China which facilitated trade and foreign relations by acknowledging China's predominant role in East Asia. It involved multiple relationships of trade, military force, diplomacy and ritual. The other states had to send a tributary envoy to China on schedule, who would kowtow to the Chinese emperor as a form of tribute, and acknowledge his superiority and precedence. The other countries followed China's formal ritual in order to keep the peace with the more powerful neighbor and be eligible for diplomatic or military help under certain conditions. Political actors within the tributary system were largely autonomous and in almost all cases virtually independent. The term "tribute system" as applied to China is technically a Western invention. There was no equivalent term in the Chinese lexicon to describe what would be considered the "tribute system" today, nor was it envisioned as an institution or system. John King Fairbank and Teng Ssu-yu created the "tribute system" theory in a series of articles in the early 1940s to describe "a set of ideas and practices developed and perpetuated by the rulers of China over many centuries." The Fairbank model presents the tribute system as an extension of the hierarchic and nonegalitarian Confucian social order.