on the eastern border of the state, the ridge-and-valley region where East Tennessee's principal cities (Knoxville, Chattanooga, and the Tri-Cities) are located, and the rugged Cumberland Mountains. East and Middle Tennessee are separated along the Cumberland Plateau. Middle Tennessee, which includes the state's capital city of Nashville, is dominated by rolling hills and fertile stream valleys. West Tennessee, located between the Tennessee and the Mississippi Rivers, is the lowest-lying of the three Grand Divisions. It is part of the Gulf Coastal Plain physiographic region, characterized by relatively flat topography. Except for the Memphis metropolitan area, land use in this region is mostly agricultural. Historically, cotton was West Tennessee's dominant crop. The physiographic and economic differences between the three regions resulted in major divisions within Tennessee in the Civil War. The plantation agricultural system associated with cotton production meant that slavery was very important to the economy of West Tennessee, where voters strongly supported secession. Support for secession was not as strong in Middle Tennessee, where plantation agriculture was present but less important. In mountainous East Tennessee, where plantation agriculture was largely absent and slavery was not economically important, voters strongly opposed secession. Although the entire state seceded from the Union and joined the Confederacy, East Tennessee remained an area of pro-Union sentiment and activity throughout the Civil War and afterward. The Republican Party dominated politics in the region. Before and during the Civil War, there was a movement in East Tennessee to counter-secede from the Confederacy and re-join the Union as the State of Nickajack, together with other Union-friendly Southern areas, such as North Alabama. Partly because of West Tennessee's history of slavery, it has had a higher concentration of African Americans in the population. In the census in 2000, West Tennessee's population was found to be 37% black, while in Middle and East Tennessee, black people made up 12% and 6% of their respective populations. Legal significance The Tennessee State Constitution mandates that no more than two of the five justices on the state's supreme court can be from any one Grand Division. The court must also meet regularly in each division in the cities of Jackson in West Tennessee, Nashville in Middle Tennessee, and Knoxville in East Tennessee. Similar rules apply to some other state institutions. For example: The former Tennessee Public Service Commission was required to consist of three commissioners, including one commissioner from each Grand Division,