All blue areas represent Union states, including those admitted during the war; light blue areas represent Union states which permitted slavery; red areas represent Confederate states.

The Border states were those states that during the American Civil War did not leave the Union. The border states were Delaware, Maryland, Kentucky, and Missouri. After West Virginia separated from Virginia, it was also considered a border state. Most border states had strong ties to the South culturally, but they had economic ties to the North. While remaining loyal to the Union, the Border States were themselves slave-holding states.