On October 10, 1767, Charles Mason and Jeremiah Dixon complete their survey of the boundary between the colonies of Pennsylvania and Maryland as well as areas that would eventually become the states of Delaware and West Virginia. The Penn and Calvert families had hired Mason and Dixon, English surveyors, to settle their dispute over the boundary between their two proprietary colonies, Pennsylvania and Maryland.

In 1760, tired of border violence between the colonies’ settlers, the British crown demanded that the parties involved hold to an agreement reached in 1732. As part of Maryland and Pennsylvania’s adherence to this royal command, Mason and Dixon were asked to determine the exact whereabouts of the boundary between the two colonies. Though both colonies claimed the area between the 39th and 40th parallel, what is now referred to as the Mason-Dixon line finally settled the boundary at a northern latitude of 39 degrees and 43 minutes. The line was marked using stones, with Pennsylvania’s crest on one side and Maryland’s on the other.

When Mason and Dixon began their endeavor in 1763, colonists were protesting the Proclamation of 1763, which was intended to prevent colonists from settling beyond the Appalachians and angering Native Americans. As the Britons concluded their survey in 1767, the colonies were engaged in a dispute with the Parliament over the Townshend Acts, which were designed to raise revenue for the empire by taxing common imports including tea.

Twenty years later, in late 1700s, the states south of the Mason-Dixon line would begin arguing for the perpetuation of slavery in the new United States while those north of line hoped to phase out the ownership of human chattel. This period, which historians consider the era of “The New Republic,” drew to a close with the Missouri Compromise of 1820, which accepted the states south of the line as slave-holding and those north of the line as free. The compromise, along with those that followed it, eventually failed.

One hundred years after Mason and Dixon began their effort to chart the boundary, soldiers from opposite sides of the line let their blood stain the fields of Gettysburg, Pennsylvania, in the Southern states’ final and fatal attempt to breach the Mason-Dixon line during the Civil War. One hundred and one years after the Britons completed their line, the United States finally admitted men of any complexion born within the nation to the rights of citizenship with the ratification of the 14th Amendment.