Sankin kotai was the system of "alternate attendance" initiated by the Tokugawa bakufu in the early 17th century to tighten their control over their daimyo vassals. Under the sankin kotai system, each lord was required to maintain an appropriately luxurious manor in Edo, under the watchful eyes of the loyal shogunal vassals, and alternate residence between Edo and his home province - at first every other year, and later every few years for a few years at a time. This clever system prevented the daimyo from become to attached to their own domains and from stirring up trouble in far-flung regions while the shogun was not paying attention.
The effectiveness of the system was further enhanced by a requirement that the wives and heirs of the daimyo remain in Edo permanently. Thus the families of the lords essentially became hostages as insurance against their good behavior.
Another way the system kept the daimyo subserviant was by draining their monetary resources. Elaborate sumptuary regulations demanded that each daimyo maintain certain standards of luxury on their journeys to and from Edo. At the zenith of the sankin kotai system, in the late 1600s, it is estimated that the average daimyo spent 25% of his annual income on his journeys to and from Edo.