Who was in the union during the Civil War?

During the American Civil War, the Union was a name used to refer to the federal government of the United States, which was supported by the twenty free states and five border slave states. It was opposed by 11 Southern slave states that had declared a secession to join together to form the Confederacy. Although the Union states included the Western states of California, Oregon, and (after 1864) Nevada, as well as states generally considered to be part of the Midwest, the Union has been also often loosely referred to as "the North", both then and now.[1]