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What is the Mason-Dixon Line?

Tricia Christensen
Updated May 17, 2024
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The Mason-Dixon Line has two definitions. One relates to the surveying of land by Charles Mason and Jeremiah Dixon from 1763-1767. The other definition refers to separation between US states by states that allowed slavery, and states which did not, or Union and Confederate States. This more common usage occurred during the Civil War.

The initial purpose of this line as drawn by Mason and Dixon was to settle disputes between people who owned land in Maryland and Pennsylvania. The boundaries of this land were in dispute and the families owning the land, the Calverts and the Penns, were contentious in regard to these borders.

Initially, The Mason-Dixon Line ran east-west through the Southern border of Pennsylvania, and north-south between the borders of Maryland and Delaware. The lines were marked by stones each at the end of each mile. Every five miles, a large crownstone was also used.

Dispute between which areas of property belonging to Delaware and Pennsylvania resulted in a 1732 agreement of specific borders. However, arguments about colony borders still existed. Finally in 1760, the King of England forced the people of both areas to accept the agreement made in 1732, and Mason and Dixon, two astronomers, were commissioned to create the line.

Later, the Mason-Dixon Line was defined as the separation between states that had seceded from the Union. The actual line, which was really symbolic in purpose, is slightly harder to define. The border states like Missouri, Kentucky, Maryland and West Virginia are sometimes considered as below the line. On other maps, the border states are north of the line.

The Mason-Dixon Line extends to Texas, which is often considered the most western of the southern states. In many ways, continued discrimination against Blacks in the southern states was seen as continued observation of the Mason-Dixon line. Though slavery was outlawed at the close of the Civil War, the Mason-Dixon line was thought of as a symbolic separation between states that continued discrimination against Blacks and those which did not to the same extent. However, it should be noted that states north of the line often proved just as terrible in their treatment and assessment of the rights of Blacks.

America Explained is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a America Explained contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
Discussion Comments
By dbuckley212 — On Jan 13, 2011

The South made the economically smart move to allow Europeans to invest in Cotton-based war bonds, but unfortunately did not invest enough naval defense in protecting their main port of New Orleans, which the navally dominant North cut off. The ultimate fate of the War may well have been determined at the moment the North squelched the Southern economic prospects and all investments from Europe.

By FitzMaurice — On Jan 10, 2011

I have heard old Southerners call the Civil War "The War of Northern Aggression." From their perspective, the North was infringing upon their rights to succeed economically via slave trade and cotton farming. The economically dominant Anglo-Saxon North was effectively flaunting its power against the largely Scots-Irish South, which it tended to look down upon, and the Southerners saw this as a challenge and an insult. It is important to see the perspective of all sides in this war.

By BostonIrish — On Jan 07, 2011

I think "discrimination against Blacks" is an understatement. They were enslaved. This is far worse than discrimination, the Southern states continued to actively harass and imprison or kill slaves who tried to escape, and were in open rebellion to the emancipation proclamation. They held to the old idea that states were to be self-determining entities which didn't need to fully accept federal laws and proclamations. When they were invaded by Union forces, they responded harshly, and split the country in a conflict.

By CarrotIsland — On Nov 19, 2010

In the year 1632, King Charles I of England gave George Calvert, the first Lord Baltimore, the colony of Maryland. In 1682, King Charles II gave William Penn the territory to the north, later becoming Pennsylvania. One year later, Charles II gave William Penn land on the Delmarva Peninsula, which was the peninsula that includes the eastern portion of our modern Maryland and all of Delaware.

The boundary description in the grants to Penn and Calvert did not match which is what led to the dispute. The Penn and Calvert families took the matter before the British court where the chief justice declared that the boundary between southern Pennsylvania and northern Maryland should lie 15 miles south of Philadelphia. It took 10 years for the two families to agree on the compromise.

Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a America Explained contributor, Tricia...
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