In the earliest years of the 19th century, approximately half the population of the U. S. lived south of the Line, which made the decision to locate the new capital city nearby an ideal one (which city soon became Washington, DC).

In the earliest years of the 19th century, approximately half the population of the U.S. lived south of the Mason-Dixon Line, which made the decision to locate the new capital city nearby an ideal one (which city soon became Washington, DC).

Explanation:

The Mason-Dixon line, named after the astronomer Charles Mason and the geodesist Jeremiah Dixon, forms the historical border between the northern and southern states of the USA. It runs essentially in the east-west direction at 39° 43' 20" north latitude. The line was surveyed between 1763 and 1767 to establish the Pennsylvania-Maryland border that still applies today.

The term "Mason-Dixon line" is still used today for the cultural border between the northern and southern states, especially with regard to the question of slavery and the Civil War.