There is no one hard and fast defining characteristic which delineates the South from other portions of the nation; there are rather several defining factors, all of which differ in some detail. The traditional definition of "the South" would be that area of the United States situate below the Mason Dixon Line. The definition came about because when slavery was still extant in the U.S., the line became de facto the dividing line between slave and free states. Another traditional definition is those States which purportedly left the Union and fought against it during the Civil War. (Under the latter definition, Maryland would not be part of the South; whereas it would be in the former.) Geographically, it might be described as that area bordered on the North by the Potomac River and on the West by the Mississippi River. Still another definition might encompass the "Black Belt" because of the rich soils which previously defined this largely agricultural area. Finally, it is often characterized as the "Bible Belt," because of the conservative, often evangelical Protestant religion which is dominant in the southern States.