What states were part of the Union during the Civil War?

There were 24 states which remained loyal to the Union during the war:

California, Connecticut, Illinois, Indiana, Iowa, Kansas, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, Ohio, Oregon, Pennsylvania, Rhode Island, Vermont, and Wisconsin.

The Union counted Virginia as well, and added Nevada and West Virginia. It added Tennessee, Louisiana, and other rebel states as soon as they were reconquered.