Missouri v. Kentucky was a landmark Supreme Court case that was decided in 1870. The case centered around a dispute between the two states over the boundary between them. Missouri argued that the boundary should be the low-water mark on the north side of the Mississippi River, while Kentucky argued that the boundary should be the high-water mark on the south side of the river. The Supreme Court ultimately sided with Missouri, ruling that the boundary should be the low-water mark on the north side of the river. The Court's decision was based on the principle of equitable apportionment, which holds that when two states are in dispute over a boundary, the boundary should be determined in a way that is fair and equitable to both states. In this case, the Court found that the low-water mark on the north side of the river was the most equitable solution, as it would give both states access to the river and its resources. The decision in Missouri v. Kentucky was significant in that it established the principle of equitable apportionment as a legal precedent. This principle has since been used in numerous other cases involving boundary disputes between states. The case also established the importance of the Mississippi River as a boundary between states, and it has since been used as a reference point in other boundary disputes.

In Missouri v. Kentucky, the Supreme Court was tasked with determining which state had jurisdiction over a disputed area of land between them. The majority opinion held that Kentucky had jurisdiction due to its possession of the land for many years prior and because it was included in their original boundaries when they became a state. However, Justice Field dissented from this decision and argued that Missouri should have been given control over the territory as they were first to claim it after becoming a state and also because Congress recognized their right to do so by passing an act granting them authority over it. He further noted that even if Kentucky did possess some rights to the area, those rights could not be superior to those granted by Congress or else there would be no point in having such laws at all. Ultimately, he concluded that since both states claimed ownership of the same piece of land simultaneously without any clear resolution from either side beforehand, then neither party should prevail until such time as one is able to prove exclusive title through legal means rather than simply relying on long-standing possession alone.