The Border States were slave states that shared a border with free states to the north. These included Delaware, Maryland, Virginia, Kentucky and Missouri. As a rule, the Border States maintained strong cultural ties with the South, but important economic relationships with the North.
All remained in the Union after the outbreak of the Civil War, except Virginia. Sentiment in that state's 40 western counties, however, was strongly against secession. In 1861 those counties seceded from Virginia forming West Virginia, which formally entered the Union in 1863.