The Mason-Dixon Line became widely known as the symbolic divider between the Northern and Southern states during America's Civil War; in short, it divided slave states from non-slave states.
However, the original Mason–Dixon Line was actually a demarcation (or border) line between Pennsylvania and Maryland, in an effort to settle an 80-year land dispute between the two colonies. It also included the western border of present-day Delaware, as it was then a part of the Pennsylvania colony.
The on-going dispute between the Penn family of Pennsylvania, and the Calvert family of Maryland over the border between the two colonies finally erupted into war in 1730, one known as Cresap's War. After years of conflict, England's King George II negotiated a cease-fire in 1738.
Shortly thereafter, the Penns and Calverts commissioned two Englishman, Charles Mason and Jeremiah Dixon, to mark the official border, and solve their property dispute.
Mason (an astronomer) and Dixon (a surveyor) used celestial measurements to form an accurate 233-mile-long line (or boundary) between Pennsylvania and Maryland, and the 83 mile-long between Maryland and Delaware.
The project took nearly 5 years, and the new border was marked by large blocks of limestone, some weighing as much as 600 pounds. Today, most of the original stone markers have either deteriorated, or simply disappeared - some say as souvenirs.
However, the Mason-Dixon Line still exists. It was last resurveyed in 1902 and found to be remarkably accurate; with minor adjustments, it still serves as the Maryland-Pennsylvania border, while the Mason-Dixon Line of the Civil War days fades into the past.