What were the states called that remained when the South left to form their own country?

The states that remained in the Union when the South seceded to form the Confederate States of America were known as the "Border States." These included Delaware, Maryland, Kentucky, and Missouri. Although they were slave states, they did not secede from the Union and played crucial roles during the Civil War. Additionally, West Virginia, which separated from Virginia in 1863, also remained loyal to the Union.