All states that are below the Mason Dixon Line?

This answer is written as if looking at the Mason Dixon Line at the beginning of the Civil War.

Florida, Georgia, Alabama, Mississippi, Louisiana, Texas

South Carolina, North Carolina, Virginia (including the future West Virginia)

Maryland

Kentucky, Tennessee, Missouri

California

Technically, the Mason-Dixon line simply established the boundary that cut between Pennsylvania, Maryland, and Delaware. It came to symbolize the cultural divide between slave economies and free economies. At the time the line was drawn, however, slavery was legal in nearly all parts of the country.

The Mason-Dixon Line is sometime confused with the line (36 degrees 30 minutes north) of the Missouri Compromise; that line was intended to limit the growth of slavery in the years leading up to the Civil War.