Toby, Ronald P. (1991). State and Diplomacy in Early Modern Japan: Asia and the development of the Tokugawa bakufu, pp. 45-46, citing manuscripts at the Historiographical Institute of the University of Tokyo; excerpt, "Ieyasu granted the Shimazu clan the right to "rule" over Ryukyu ... [and] contemporary Japanese even referred to the Shimazu clan as 'lords of four provinces', which could only mean that they were including the Ryukyuan kingdom in their calculations. However, this does not mean that Ryukyu ceased to be a foreign country or that relations between Naha and Edo ceased thereby to be foreign relations."
